A simple pendulum is vibrating with an angular amplitude of $\frac{\pi}{2}$. The value of $\alpha$ for which the resultant acceleration has a direction along the horizontal is
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(a) $\frac{\pi}{2}$
(b) $180^{\circ}$
(c) $\cos ^{-1}\left(\frac{1}{\sqrt{3}}\right)$
(d) $\cos ^{-1}\left(\frac{1}{\sqrt{2}}\right)$