6.(5%) Which of the following is true?
(A) Every nonzero subspace of Rn has a unique basis.
(B) Every subspace of Rn has a basis composed of standard vectors.
(C) The column space of an m ✖ n matrix is contained in Rm.
(D) If V is a subspace of dimension k, then every generating set for V contains exactly hvectors.
(E) The pivot columns of a m ✖ n matrix A form a basis for the column space of A.