The equaion of the line common to the pairs of lines <br> $(p^2 - q^2)x^2 + (q^2 - r^2) xy + (r^2 - p^2) y^2 = 0$ and $(f-m)x^2 + (m-n)xy + (n-f)y^2 = 0$ is :

Question

( A ) $x-y=0$
( B ) $x-y=pqr$
( C ) $x+y= pqr$
( D ) $x+y=0$