Which of the following planes are perpendicular to one another? (a) 2x + 3y - 5z = 4 (b) 2x-3y-z=3 (c) 4x+6y-2z=0 (d) z=4-2x-3y (e) z= 4+2x+3y (f) x+y+z=1 (a) and (b) (a) and (f) (b) and (f) (c) and (d) (c) and (e)