Marginal Rate of Substitution: MRS of X for Y refers to the number of units of good Y that the consumer is willing to forego for an additional unit of good X, so as to maintain the same level of satisfaction. Indifference curve is convex to origin due to diminishing marginal rate of substitution. MRS falls because when more and more units of X are obtained, its marginal utility declines and the consumer likes to sacrifice less of Y to obtain an additional unit of X. So, MRS declines.