The two lines $x=a y+b, z=c y+d ;$ and $x=a^{\prime} y+b^{\prime}, z$ $=c^{\prime} y+d^{\prime}$ are perpendicular to each other if $[\mathbf{2 0 0 6}]$
(A) $a a^{\prime}+c c^{\prime}=-1$
(B) $a a^{\prime}+c c^{\prime}=1$
(C) $\frac{a}{a^{\prime}}+\frac{c}{c^{\prime}}=-1$