The differential equation for the family of curves $x^2 + y^2 - 2ay = 0$, where $a$ is an arbitary constant is

Question

( A ) $2(x^2- y^2)y' = xy$
( B ) $(x^2+ y^2)y' = 2xy$
( C ) $(x^2- y^2)y' = 2xy$
( D ) $2(x^2+ y^2)y' = xy$