$A$ container of mass $m$ is pulled by a constant force $F$. Inside the container,a second block of the same mass $m$ is placed,connected to the wall by a massless spring of constant $k$. Initially,the spring is at its natural length. Find the velocity of the container at the instant the compression in the spring is maximum for the first time.

• A
  $\pi F \sqrt{\frac{1}{2km}}$
• B
  $\frac{\pi F}{2} \sqrt{\frac{1}{2km}}$
• C
  $\pi F \sqrt{\frac{1}{km}}$
• D
  $\frac{\pi F}{2} \sqrt{\frac{1}{km}}$