Robinson Spends All His Income on Mangos and Bananas

Robinson spends all his income on mangos and bananas. Mangos cost $3 per pound and bananas cost $1 per pound. The marginal utility is 30 for the last pound of mangos purchased and 10 for the last pound of bananas. To maximize his utility, Robinson should buy

A) more mangos and fewer bananas.
B) more bananas and fewer mangos.
C) the present combination of goods.
D) only mangos.