Grayson's Marginal Utility Per Dollar Spent on Bananas Is 10

Grayson's marginal utility per dollar spent on bananas is 10 and his marginal utility per dollar spent on oranges is 20, how should he spend his income?

A) He should purchase fewer oranges and more bananas.
B) He should purchase an equal amount of oranges and bananas.
C) He should purchase more oranges and fewer bananas.
D) He should purchase more bananas and oranges.