Refer to the Indifference Curve in the Diagram Above

Refer to the indifference curve in the diagram above.The marginal rate of substitution is equal to

A) the rate at which apples can be substituted for bananas without changing total satisfaction.
B) the rate at which bananas can be substituted for apples without changing total satisfaction.
C) the rate at which apples can be substituted for bananas such that total satisfaction will be increased.
D) the rate at which apples can be substituted for bananas such that total satisfaction will be decreased.
E) the rate at which bananas can be substituted for apples such that total satisfaction will be increased.