Time left 2:53:26 A consumer's utility function is U(X1,X2,X3,X4)=3X1+X1X2+X3X4-X1X5 . How does the number of units of good 1 that the consumer is willing to give up in order to obtain an additional unit of good 3 change if only his consumption x5 of good 5 changes? a. It stays the same. O. b. It goes down if X5 goes up. O. c. None of the other answers. O d. It goes up if X5 goes up