If AB = O, then for the matrices \[A=\left[ \begin{matrix} {{\cos }^{2}}\theta & \cos \theta \sin \theta \\ \cos \theta \sin \theta & {{\sin }^{2}}\theta \\ \end{matrix} \right]\] and \[B=\left[ \begin{matrix} {{\cos }^{2}}\phi & \cos \phi \sin \phi \\ \cos \phi \sin \phi & {{\sin }^{2}}\phi \\ \end{matrix} \right],\theta -\phi \] is

• A. An odd number of \[\frac{\pi }{2}\]
• B. An odd multiple of \[\pi \]
• C. An even multiple of \[\frac{\pi }{2}\]
• D.             0