The value of $\cos y\cos\big(\large\frac{\pi}{2}$$-x)$$-\cos(\large\frac{\pi}{2}$$-y\big)\cos x+\sin y\cos\big(\large\frac{\pi}{2}-x\big)$$+\cos x.\sin\big(\large\frac{\pi}{2}$$-y\big)$ is zero.If

Question

$(a)\;x=0\qquad(b)\;y=0\qquad(c)\;x=y\qquad(d)\;x=n\pi-\large\frac{\pi}{4}$$+y$