18. Let \( A=\left[\begin{array}{cc}\cos ^{2} \theta & \sin \theta \cos \theta \\ \cos \theta \sin \theta & \sin ^{2} \theta\end{array}\right] \) and \( B=\left[\begin{array}{cc}\cos ^{2} \phi & \sin \phi \cos \phi \\ \cos \phi \sin \phi & \sin ^{2} \phi\end{array}\right] \), then \( A B=O \), if
(a) \( \theta=n \phi, n=0,1,2, \ldots . . \)
(b) \( \theta+\phi=n \pi, n=0,1,2, \ldots . . \)
(c) \( \theta=\phi+(2 n+1) \frac{\pi}{2}, n=0,1,2, \ldots . . \)
(d) \( \theta=\phi+\frac{n \pi}{2}, n=0,1,2, \ldots . . \)
