Determine whether the following sets form subspaces of R2. (a) { (x1, x2) | x1+ x2= 0} (b) { (x1, x2) | x1x2=0) no (c) {(x1, X2) | x1 = 3x2) (d) { ( x1, x2) | |xil = 1x2/} no e) { ( x1, x2) ? | x2 = x2)