A particle of mass $200\;g$ is dropped from a height of $50\;m$ and another particle of mass $100\; g$ is simultaneously projected up from the ground along the same line with a speed of $100\; m/s$. The acceleration of the centre of mass after $1\; s$ is

Question

$$\begin {array} {1 1} (a)\;10\;m/s & \quad (b)\;\frac{10}{3}\;m/s^2 \\ (c)\;0 & \quad  (d)\;none\;of\;these \end {array}$$