10. (5%) Which of the following is a vector space?
(A) P = {p I p is a polynomial in z of any order}
(B) Pn = {p | p is a polynomial in ； of exactly nth order}
(C) {TIT : Rn → Rm,T is a linear transformation}
(D) The set of all m x n n matrices of which the sum of all entries is zero,ie,