# The differential equation for the family of curves $x^2+y^2-2ay=0$ where a is an arbitary constant is given $y'= \large\frac{dy}{dx}$
$(a)\;(x^2-y^2)y'=2 xy \\ (b)\;2 (x^2+y^2)y'=xy \\ (c)\;2(x^2-y^2)y'=xy \\ (d)\;(x^2+y^2)y'=2xy$