can someone help me correct this

Directions: Dtermine which two equations represent perpendicular lines.

options are:

A) y = 2x-4
b) y = (1)/(3) x + 4
c) y= - (1)/(2)x +4
d) y = (1)/(3)x - 4

wouldn't it be a and c . Because b and d have the dame slope which are parallel lines. am i correct or wrong.