: While the latter is always negative, the former can be either positive or negative.When the price of x rises, Marvin responds by changing his demand for x. The substitution effect is the part of this change that represents his change in demand:A:if he is allowed to substitute as much x for y as he wishes. B:none of all. C:holding the prices of substitutes constant. D:if his money income is held constant when the price of x changes.