Under which one of the following conditions are the lines x = ay + b; z = cy + d and x = ey + f; z = gy + h perpendicular?

Under which one of the following conditions are the lines x = ay + b;

1.	Under which one of the following conditions are the lines x = ay + b; z = cy + d and x = ey + f; z = gy + h perpendicular?
A.	ae + cg – 1 = 0
B.	ae + bf – 1 = 0
C.	ae + cg + 1 = 0
D.	ag + ce + 1 = 0
Answer» D. ag + ce + 1 = 0