In an Indifference Curve Diagram, the Quantities of Good Y

In an indifference curve diagram, the quantities of good Y are measured along the vertical axis and the quantities of good X are measured along the horizontal axis. The marginal rate of substitution is defined as

A) how much good Y you must give up to get one more unit of good X.
B) how much good Y you are willing to give up to get one more unit of good X.
C) how much you prefer to substitute good X for good Y.
D) the relative price of good Y in terms of good X.