Gigi Consumes Only Perrier and Cheese

Gigi consumes only Perrier and cheese. In order to maximize her happiness subject to a limited income, Gigi should purchase the amounts of Perrier and cheese at which

A) the addition to happiness of the last bottle of Perrier is the same as the addition to happiness of the last pound of cheese.
B) the addition to happiness per dollar spent on Perrier is the same as the addition to happiness per dollar spent on cheese.
C) she spends all her income.
D) both b and c
E) both a and c