Which of the following are equations of perpendicular lines?

a. y= -1/2x+3 , y=-2x+3

b. y= -1/2x+3 , y= 2x-3

c. y= -1/2x+3 , y= -1/2x - 1

d. y= - 1/2x+3 , y= - 2x+1