Matrix dimensions. Suppose A is an m×n matrix, and B is a p×q matrix. For each of the following equations, give the conditions on m, n, p, q under which it makes sense. For example, A = B requires that m = p and n = q. (a) ATB = I. (b) AB = 0. (c) AB = BA. (d) AT = [B I]. (e) [A B] = I.