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 prinicple 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.