A ball A of mass m falls under gravity from a height h and strikes another ball B of mass m which is supported at rest on a spring of stiffness k. Assume perfect elastic impact. Immediately after the impact.
(a) the velocity of ball A is $\frac{1}{2}\sqrt{2gh}$ 
(b) the velocity of ball A” is zero
(c) the velocity of both ball is $\frac{1}{2}\sqrt{2gh}$ 
(d) none of the above