$A$ block of mass $m$ is connected to another block of mass $M$ by a massless spring of spring constant $k$. The blocks are kept on a smooth horizontal plane. Initially,the blocks are at rest and the spring is unstretched. Then a constant force $F$ starts acting on the block of mass $M$ to pull it. Find the force acting on the block of mass $m$.

• A
  $\frac{MF}{m + M}$
• B
  $\frac{mF}{M}$
• C
  $\frac{(m + M)F}{m}$
• D
  $\frac{mF}{m + M}$