Why Do Convex Indifference Curves Have a Negative Slope

Multiple Choice

Why do convex indifference curves have a negative slope?

A) because consumers take market prices as given
B) because consumers face a budget constraint
C) because to keep utility constant, a consumer must get more of one good if she is to give up some of the other
D) because scarcity implies that it is not possible to consume more of one good without giving up some of the other