If \[\sin y=x\cos (a+y),\] then \[\frac{dy}{dx}=\]

• A.            \[\frac{{{\cos }^{2}}(a+y)}{\cos a}\]
• B.            \[\frac{\cos (a+y)}{{{\cos }^{2}}a}\]
• C.            \[\frac{{{\sin }^{2}}(a+y)}{\sin a}\]
• D.            None of these