Two blocks of masses $6\; kg$ and $4\;kg$ are attached to the two ends of a massless string passing over a smooth fixed pulley. If the system is released, the acceleration of the centre of mass of the system will be ;

Question

$\begin{array}{1 1} a) g_1\; vertically\; downwards \\ b) \frac{g}{5}, vertically\; downwards \\ c) \frac{g}{25}, vertically\; downwards \\ d) zero \end{array}$