$A$ large ball of mass $M$,moving with velocity $u$,strikes a small ball of mass $m$,which is at rest. After the collision,the small ball attains velocity $u$ and the large ball attains velocity $v$. What is the value of $v$?

• A
  $\frac{M - m}{M}u$
• B
  $\frac{m}{M + m}u$
• C
  $\frac{2m}{M + m}u$
• D
  $\frac{M}{M + m}u$