If Charles Darwin could have seen the results of this experiment, his most probable response would have been one of

Pigeons have been taught to recognize human facial expressions, upsetting long-held beliefs that only humans had evolved the sophisticated nervous systems to perform such a feat. In recent experiments at the University of Iowa, eight trained pigeons were shown photographs of people displaying emotions of happiness, anger, surprise, and disgust. The birds learned to distinguish between these expressions. Not only that, but they were also able to correctly identify the same expressions on photographs of unfamiliar faces. Their achievement does not suggest, of course, that the pigeons had any idea what the human expressions meant.

Some psychologists have theorized that because of the importance of facial expression to human communication, humans developed special nervous systems capable of recognizing subtle expressions. The pigeons cast doubt on that idea, however.

In fact, the ability to recognize facial expressions of emotion is not necessarily innate even in human babies, but may have to be learned in much the same way pigeons learn. In experiments conducted several years ago at the University of Iowa, it was found that pigeons organize images of things into the same logical categories that humans do.

None of this work would come as any surprise to Charles Darwin, who long ago wrote about the continuity of mental development from animals to humans.

Why does the author mention a rainbow in paragraph 1?

The spectacular and famous eruptions of Old Faithful geyser in Yellowstone National Park do not occur like clockwork. Before the earthquake of 1959, eruptions came every 60 to 65 minutes; today they are as little as 30 minutes or as much as 90 minutes apart. Changes in weather and in atmospheric pressure can influence the regularity of the eruptions and the height of the column. The geyser usually gives a warning: a short burst of steam. Then a graceful jet of water and steam rises up to 150 feet in the air, unfurling in the sunlight with the colors of the rainbow playing across it.

The eruption is only the visible part of the spectacle. In order for a geyser to erupt, there are three necessary ingredients: a heat source, a water supply, and a plumbing system. In the geyser fields of Yellowstone, a steady supply of heat is provided by hot spots of molten rock as little as two miles below the surface. The water supply of Old Faithful comes from groundwater and rainfall, but other geysers in Yellowstone are located on river banks. Geysers have various types of plumbing systems. Geologists studying Old Faithful theorized that it had a relatively simple one consisting of an underground reservoir connected to the surface by a long, narrow tube. In 1992 a probe equipped with a video camera and heat sensors was lowered into the geyser and confirmed the existence of a deep, narrow shaft and of a cavern, about the size of a large automobile, about 45 feet beneath the surface.

As water seeps into Old Faithful’s underground system, it is heated at the bottom like water in a teakettle. But while water in a kettle rises because of convection, the narrow tube of the plumbing system prevents free circulation. Thus, the water in the upper tube is far cooler than the water at the bottom. The weight of the water puts pressure on the column, and this raises the boiling point of the water near the bottom. Finally, the confined, superheated water rises, and the water in the upper part of the column warms and expands, some of it welling out of the mouth of the geyser. This abruptly decreases the pressure on the superheated water, and sudden, violent boiling occurs throughout much of the length of the tube, producing a tremendous amount of steam and forcing the water out of the vent in a superheated mass. This is the eruption, and it continues until the water reservoir is emptied or the steam runs out.

There are two main types of geysers. A fountain geyser shoots water out in various directions through a pool. A columnar geyser such as Old Faithful shoots water in a fairly narrow jet from a conical formation at the mouth of the geyser that looks like a miniature volcano.

It can be inferred from the passage that which of the following would be LEAST likely to cause any change in Old Faithful’s eruptions?

The spectacular and famous eruptions of Old Faithful geyser in Yellowstone National Park do not occur like clockwork. Before the earthquake of 1959, eruptions came every 60 to 65 minutes; today they are as little as 30 minutes or as much as 90 minutes apart. Changes in weather and in atmospheric pressure can influence the regularity of the eruptions and the height of the column. The geyser usually gives a warning: a short burst of steam. Then a graceful jet of water and steam rises up to 150 feet in the air, unfurling in the sunlight with the colors of the rainbow playing across it.

The eruption is only the visible part of the spectacle. In order for a geyser to erupt, there are three necessary ingredients: a heat source, a water supply, and a plumbing system. In the geyser fields of Yellowstone, a steady supply of heat is provided by hot spots of molten rock as little as two miles below the surface. The water supply of Old Faithful comes from groundwater and rainfall, but other geysers in Yellowstone are located on river banks. Geysers have various types of plumbing systems. Geologists studying Old Faithful theorized that it had a relatively simple one consisting of an underground reservoir connected to the surface by a long, narrow tube. In 1992 a probe equipped with a video camera and heat sensors was lowered into the geyser and confirmed the existence of a deep, narrow shaft and of a cavern, about the size of a large automobile, about 45 feet beneath the surface.

As water seeps into Old Faithful’s underground system, it is heated at the bottom like water in a teakettle. But while water in a kettle rises because of convection, the narrow tube of the plumbing system prevents free circulation. Thus, the water in the upper tube is far cooler than the water at the bottom. The weight of the water puts pressure on the column, and this raises the boiling point of the water near the bottom. Finally, the confined, superheated water rises, and the water in the upper part of the column warms and expands, some of it welling out of the mouth of the geyser. This abruptly decreases the pressure on the superheated water, and sudden, violent boiling occurs throughout much of the length of the tube, producing a tremendous amount of steam and forcing the water out of the vent in a superheated mass. This is the eruption, and it continues until the water reservoir is emptied or the steam runs out.

There are two main types of geysers. A fountain geyser shoots water out in various directions through a pool. A columnar geyser such as Old Faithful shoots water in a fairly narrow jet from a conical formation at the mouth of the geyser that looks like a miniature volcano.

The passage implies that Old Faithful would probably not erupt at all if

The spectacular and famous eruptions of Old Faithful geyser in Yellowstone National Park do not occur like clockwork. Before the earthquake of 1959, eruptions came every 60 to 65 minutes; today they are as little as 30 minutes or as much as 90 minutes apart. Changes in weather and in atmospheric pressure can influence the regularity of the eruptions and the height of the column. The geyser usually gives a warning: a short burst of steam. Then a graceful jet of water and steam rises up to 150 feet in the air, unfurling in the sunlight with the colors of the rainbow playing across it.

The eruption is only the visible part of the spectacle. In order for a geyser to erupt, there are three necessary ingredients: a heat source, a water supply, and a plumbing system. In the geyser fields of Yellowstone, a steady supply of heat is provided by hot spots of molten rock as little as two miles below the surface. The water supply of Old Faithful comes from groundwater and rainfall, but other geysers in Yellowstone are located on river banks. Geysers have various types of plumbing systems. Geologists studying Old Faithful theorized that it had a relatively simple one consisting of an underground reservoir connected to the surface by a long, narrow tube. In 1992 a probe equipped with a video camera and heat sensors was lowered into the geyser and confirmed the existence of a deep, narrow shaft and of a cavern, about the size of a large automobile, about 45 feet beneath the surface.

As water seeps into Old Faithful’s underground system, it is heated at the bottom like water in a teakettle. But while water in a kettle rises because of convection, the narrow tube of the plumbing system prevents free circulation. Thus, the water in the upper tube is far cooler than the water at the bottom. The weight of the water puts pressure on the column, and this raises the boiling point of the water near the bottom. Finally, the confined, superheated water rises, and the water in the upper part of the column warms and expands, some of it welling out of the mouth of the geyser. This abruptly decreases the pressure on the superheated water, and sudden, violent boiling occurs throughout much of the length of the tube, producing a tremendous amount of steam and forcing the water out of the vent in a superheated mass. This is the eruption, and it continues until the water reservoir is emptied or the steam runs out.

There are two main types of geysers. A fountain geyser shoots water out in various directions through a pool. A columnar geyser such as Old Faithful shoots water in a fairly narrow jet from a conical formation at the mouth of the geyser that looks like a miniature volcano.

The author implies that, compared to Old Faithful, many other geysers

The spectacular and famous eruptions of Old Faithful geyser in Yellowstone National Park do not occur like clockwork. Before the earthquake of 1959, eruptions came every 60 to 65 minutes; today they are as little as 30 minutes or as much as 90 minutes apart. Changes in weather and in atmospheric pressure can influence the regularity of the eruptions and the height of the column. The geyser usually gives a warning: a short burst of steam. Then a graceful jet of water and steam rises up to 150 feet in the air, unfurling in the sunlight with the colors of the rainbow playing across it.

The eruption is only the visible part of the spectacle. In order for a geyser to erupt, there are three necessary ingredients: a heat source, a water supply, and a plumbing system. In the geyser fields of Yellowstone, a steady supply of heat is provided by hot spots of molten rock as little as two miles below the surface. The water supply of Old Faithful comes from groundwater and rainfall, but other geysers in Yellowstone are located on river banks. Geysers have various types of plumbing systems. Geologists studying Old Faithful theorized that it had a relatively simple one consisting of an underground reservoir connected to the surface by a long, narrow tube. In 1992 a probe equipped with a video camera and heat sensors was lowered into the geyser and confirmed the existence of a deep, narrow shaft and of a cavern, about the size of a large automobile, about 45 feet beneath the surface.

As water seeps into Old Faithful’s underground system, it is heated at the bottom like water in a teakettle. But while water in a kettle rises because of convection, the narrow tube of the plumbing system prevents free circulation. Thus, the water in the upper tube is far cooler than the water at the bottom. The weight of the water puts pressure on the column, and this raises the boiling point of the water near the bottom. Finally, the confined, superheated water rises, and the water in the upper part of the column warms and expands, some of it welling out of the mouth of the geyser. This abruptly decreases the pressure on the superheated water, and sudden, violent boiling occurs throughout much of the length of the tube, producing a tremendous amount of steam and forcing the water out of the vent in a superheated mass. This is the eruption, and it continues until the water reservoir is emptied or the steam runs out.

There are two main types of geysers. A fountain geyser shoots water out in various directions through a pool. A columnar geyser such as Old Faithful shoots water in a fairly narrow jet from a conical formation at the mouth of the geyser that looks like a miniature volcano.

The author mentions the probe that was lowered into Old Faithful in 1992 to indicate that

The spectacular and famous eruptions of Old Faithful geyser in Yellowstone National Park do not occur like clockwork. Before the earthquake of 1959, eruptions came every 60 to 65 minutes; today they are as little as 30 minutes or as much as 90 minutes apart. Changes in weather and in atmospheric pressure can influence the regularity of the eruptions and the height of the column. The geyser usually gives a warning: a short burst of steam. Then a graceful jet of water and steam rises up to 150 feet in the air, unfurling in the sunlight with the colors of the rainbow playing across it.

The eruption is only the visible part of the spectacle. In order for a geyser to erupt, there are three necessary ingredients: a heat source, a water supply, and a plumbing system. In the geyser fields of Yellowstone, a steady supply of heat is provided by hot spots of molten rock as little as two miles below the surface. The water supply of Old Faithful comes from groundwater and rainfall, but other geysers in Yellowstone are located on river banks. Geysers have various types of plumbing systems. Geologists studying Old Faithful theorized that it had a relatively simple one consisting of an underground reservoir connected to the surface by a long, narrow tube. In 1992 a probe equipped with a video camera and heat sensors was lowered into the geyser and confirmed the existence of a deep, narrow shaft and of a cavern, about the size of a large automobile, about 45 feet beneath the surface.

As water seeps into Old Faithful’s underground system, it is heated at the bottom like water in a teakettle. But while water in a kettle rises because of convection, the narrow tube of the plumbing system prevents free circulation. Thus, the water in the upper tube is far cooler than the water at the bottom. The weight of the water puts pressure on the column, and this raises the boiling point of the water near the bottom. Finally, the confined, superheated water rises, and the water in the upper part of the column warms and expands, some of it welling out of the mouth of the geyser. This abruptly decreases the pressure on the superheated water, and sudden, violent boiling occurs throughout much of the length of the tube, producing a tremendous amount of steam and forcing the water out of the vent in a superheated mass. This is the eruption, and it continues until the water reservoir is emptied or the steam runs out.

There are two main types of geysers. A fountain geyser shoots water out in various directions through a pool. A columnar geyser such as Old Faithful shoots water in a fairly narrow jet from a conical formation at the mouth of the geyser that looks like a miniature volcano.

The author probably compares the formation at the mouth of Old Faithful with a volcano because of the formation’s

The spectacular and famous eruptions of Old Faithful geyser in Yellowstone National Park do not occur like clockwork. Before the earthquake of 1959, eruptions came every 60 to 65 minutes; today they are as little as 30 minutes or as much as 90 minutes apart. Changes in weather and in atmospheric pressure can influence the regularity of the eruptions and the height of the column. The geyser usually gives a warning: a short burst of steam. Then a graceful jet of water and steam rises up to 150 feet in the air, unfurling in the sunlight with the colors of the rainbow playing across it.

The eruption is only the visible part of the spectacle. In order for a geyser to erupt, there are three necessary ingredients: a heat source, a water supply, and a plumbing system. In the geyser fields of Yellowstone, a steady supply of heat is provided by hot spots of molten rock as little as two miles below the surface. The water supply of Old Faithful comes from groundwater and rainfall, but other geysers in Yellowstone are located on river banks. Geysers have various types of plumbing systems. Geologists studying Old Faithful theorized that it had a relatively simple one consisting of an underground reservoir connected to the surface by a long, narrow tube. In 1992 a probe equipped with a video camera and heat sensors was lowered into the geyser and confirmed the existence of a deep, narrow shaft and of a cavern, about the size of a large automobile, about 45 feet beneath the surface.

As water seeps into Old Faithful’s underground system, it is heated at the bottom like water in a teakettle. But while water in a kettle rises because of convection, the narrow tube of the plumbing system prevents free circulation. Thus, the water in the upper tube is far cooler than the water at the bottom. The weight of the water puts pressure on the column, and this raises the boiling point of the water near the bottom. Finally, the confined, superheated water rises, and the water in the upper part of the column warms and expands, some of it welling out of the mouth of the geyser. This abruptly decreases the pressure on the superheated water, and sudden, violent boiling occurs throughout much of the length of the tube, producing a tremendous amount of steam and forcing the water out of the vent in a superheated mass. This is the eruption, and it continues until the water reservoir is emptied or the steam runs out.

There are two main types of geysers. A fountain geyser shoots water out in various directions through a pool. A columnar geyser such as Old Faithful shoots water in a fairly narrow jet from a conical formation at the mouth of the geyser that looks like a miniature volcano.