Select the parallel lines from the given pairs of lines :

Select one:

a.
y = x / 3 + 1, y = - x / 2 + 15


b.
y = 7x + 11, y = x / 2 + 11


c.
y = 3x + 21, y = − 3x - 8


d.
y = 5x / 2 + 1, y = 5x / 2 + 7