If $\cos x = a$ where $\frac{\pi}{2} < x < \pi$, and $\cos y = -a$, then which of the following could be the value of $y$ ?

Question

$\begin{array}{1 1} (a)\; x + 2 \pi \\ (b)\; x + \pi \\ (c)\; x + \frac{\pi}{2} \\ (d)\; -x + 2 \pi \end{array}$