# The two lines $x=ay + b, \; z = cy + d$ and $x = a' y + b', \; z = c'y + d'$ are perpendicular to each other, if :
( A ) $aa' + cc' = 1$
( B ) $\frac{a}{a'} + \frac{c}{c'} = -1$
( C ) $aa' + cc' = -1$
( D ) $\frac{a}{a'} + \frac{c}{c'} = 1$