$u = \cos^{-1} (\frac{x+y}{\sqrt{x} + \sqrt{y}})$, then $x \frac{\partial u}{\partial x} + y \frac{\partial u}{\partial y}$ is equal to :

Question

( A ) $2 \cot u$
( B ) $-\frac{1}{2} \cot u$
( C ) $\frac{1}{2} \cot u$
( D ) $3 \cot u$