An individual's expenditure function is given by E(p,U)=p1p2U, where p=(p1,p2) is the price vector, U is her utility, and (,) are parameters. For this individual: (a) consumer surplus is a correct measure of welfare change for all and (b) consumer surplus is an incorrect measure of welfare for all and (c) consumer surplus is a correct measure of welfare change if and only if = (d) consumer surplus is the correct measure of welfare change for all if and only if > 0 and >0