The Marginal Rate of Substitution

The marginal rate of substitution

A) is the amount of one good the consumer is willing to give up in exchange for another so as to keep total satisfaction unchanged.
B) is constant as one moves along a particular indifference curve.
C) is the amount of one good the consumer is willing to give up in exchange for another so as to keep total expenditure unchanged.
D) is equal to the price ratio on the budget line.
E) always has a positive algebraic value.