$\sin^2 \theta = \frac{4xy}{(x+y)^2}$ is true if and only is :

Question

( A ) $ x + y \neq 0$
( B ) $x=y$
( C ) $x = y, \; x \neq 0, \; y \neq 0$
( D ) $ x \neq 0, \; y \neq 0$