If You Decide to Go to Cancún with Your Friends

Multiple Choice

If you decide to go to Cancún with your friends during spring break,you cannot go to Paris with your sister in the summer.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.