Which of the following are subspaces of P3? Support your answer.
(a) U = {f(x) | f(x) in P3, f(2) = 1}
(b) U= {xg(x) + (1 - x)h(x) | g(x) and h(x) in P2}
(c) U = {f(x) | f(x) in P3, degf(x) = 3}