True or false? justify your answers! (a) let a be an m xn-matrix and b be an m x l-matrix. then the product ab is defined. (b) the weights c1, cp in a linear combination civi + + cup cannot all be zero. (c) span{u, v} contains the line through u and the origin. (d) asking whether the linear system corresponding to [ ai a2 a3 b) is consistent, is the same as asking whether b is a linear combination of a1, a2, a3. 105