Grades Are Low in Class and Your Professor Makes Available

Multiple Choice

Grades are low in class and your professor makes available a 10-point extra credit assignment. Most of the students turn in the assignment. This statement best represents this economic concept:

A) The real cost of something is what you must give up to get it.
B) "How much" is a decision at the margin.
C) People usually exploit opportunities to make themselves better off.
D) There are gains from trade.