A ball is moving with velocity v strikes a wall moving towards the ball with velociy u. An elastic impact lasts for t seconds. Find the mean elastic force acting on the ball

Question

$$(a)\;\frac{2m(v-u)}{t} \quad (b)\; \frac{2m(v+u)}{t} \quad (c)\;\frac{2m(v+u)}{t(v-u)}\quad(d)\;\frac{m(v+u)}{t}$$