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Great quantities of water circulate either temporarily ________ in a concentrated flow of streams and rivers.
Astronauts are expected to be capable of operating at peak efficiency in outer space in both normal _________ emergency situations.
An astronomical observatory is a station--either a building _________ a group of building--used for the study of the heavens.
In commensalism, tiny plants live out their lives in the human body and derive their food from it, but probably neither harm nor _______ the host in any way.
Plants require both large amounts of carbon, oxygen and hydrogen ____ smaller quantities of phosphorus, potassium and calcium.