$A$ ball of mass $m = 60 \text{ g}$ is shot with speed $v_0 = 22 \text{ m/s}$ into the barrel of a spring gun of mass $M = 240 \text{ g}$,which is initially at rest on a frictionless surface. The ball sticks in the barrel at the point of maximum compression of the spring. The speed of the spring gun after the ball stops relative to the barrel is:

• A
  $2.2 \text{ m/s}$
• B
  $4.4 \text{ m/s}$
• C
  $4.4 \text{ cm/s}$
• D
  None of these