Indicate whether each statement is always true or sometimes false. Justify your answer by giving a logical argument or a counterexample.
(a) If Ax = b is any consistent linar system of m equations in n unknowns, then the solution set is a subspace of Rn.
(b) If W is a set of one or more vectors from a vector space V, and if ku + v is a vector in W for all vectors u and v in W and for all scalars k, then W is a subspace of V.
(c) The intersection of two subspaces of a vector space V is also a subspace of V.
(d) If span(S1) = span (S2), then = S1 = S2.