Grades Are Low in Your Biology Class,so Your Professor Offers

Multiple Choice

Grades are low in your biology class,so your professor offers an optional 10-point extra credit assignment.Most of the students turn in the assignment.Which economic principle does this statement BEST represent?

A) The true cost of something is its opportunity cost.
B) "How much" is a decision at the margin.
C) People usually respond to incentives,exploiting opportunities to make themselves better off.
D) There are gains from trade.