Two identical billiard balls are in contact on a smooth table. A third identical ball strikes them symmetrically and come to rest after impact. The coefficient of restitution is

Question

$$\begin {array} {1 1} (a)\;\frac{2}{3} & \quad (b)\; \frac{1}{2} \\ (c)\;\frac{1}{3} & \quad  (d)\;\sqrt {\frac{2}{3}} \end {array}$$