4. Assume that a consumer has utility function u : R4 -> R given by u(x1, x2) = (21+1)(x2+1). (a) Show that u is quasiconcave. Show (by finding a counterexample) that u is not a concave function. (b) Explain why the preference generated by u is not the same as the preference generated by U(21, X2) = (21 + 1)12. How about the preference generated by v($1, 12) = min {(21 + 1)(x2 + 1),9}