Robinson Spends All His Income on Mangos and Bananas

Robinson spends all his income on mangos and bananas. Mangos cost $3 per pound. Robinson's marginal utility is 30 for the last pound of mangos purchased and 10 for the last pound of bananas. If Robinson maximizes his utility from consuming these goods, the price of bananas is

A) $0.50 per pound.
B) $1 per pound.
C) $2 per pound.
D) $3 per pound.