The marginal rate of substitution (MRS) of X for Y in consumption refers to the:
选项：

A:amount of X that a nation must give up for one extra unit of Y and still remain on thesame indifference curve;
B:amount of Y that a nation must give up for one extra unit of X and still remain on the same indifference curve;
C:amount of X that a nation must give up for one extra unit of Y to reach a higherindifference curve;
D:amount of Y that a nation must give up for one extra unit of X to reach a higherindifference curve