A block of mass $m$ is connected to another block of mass $M$ by a spring (massless) 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 on the block of mass $m$:

1. A
   $\frac{mF}{M}$
2. B
   $\frac{(M+m)F}{m}$
3. C
   $\frac{mF}{m+M}$
4. D
   $\frac{MF}{m+M}$