What is opportunity cost?

What is Michael's opportunity cost when he chooses to go fishing with his friend instead of attending a concert?

What is the opportunity cost for Andrea's daughter who decides to go to the beach instead of the mountains?

What is Jason's opportunity cost when he chooses to ride 4-wheelers with his son instead of working overtime for his boss?

Ashton tried out for cheerleading and made the varsity team. She can choose to cheer for football or basketball. She chooses to cheer for football, what is her opportunity cost?

What is Aiken's opportunity cost if he chooses to ride SeeNo instead of Frappe at the horse show?

Lydia brought a salad for lunch today. Michelle is ordering pizza and asks Lydia if she would like to order. Lydia decides to eat her salad, what is her opportunity cost?