If the velocity of a particle is given by $v= (180-15x)^{1/2}$. Then its acceleration will be

Question

$\begin{array}{1 1} (a) \;zero \\ (b) \;8\;m/s^2\\ (c) \;-8\;m/s^2 \\ (d) \;4\;m/s^2\end{array}$