(3 marks) Below are sets, containing three functions each. For which set are all three functions eigenfunctions for the BVP y" + Ay =0, y'(0) =0, y'(x) =0? (a) {2, sin(xx), sin(-3xx)} (b) {cosc/2, cos(-3.c/2), cos(5x/2)} (c) {sin(3mx/2), sin(-x/2), sin(7mx/2) } (d) {4, cos(-I), cos(5x)} (e) {sin(x/2), cos(x/2), sin(3x/2)}