An isolated rail car of mass $M$ is moving along a straight,frictionless track at an initial speed $v_0$. The car is passing under a bridge when a crate filled with $N$ bowling balls,each of mass $m$,is dropped from the bridge into the bed of the rail car. The crate splits open and the bowling balls bounce around inside the rail car,but none of them fall out. Is the momentum of the rail car $+$ bowling balls system conserved in this collision?

• A
  Yes,the momentum is completely conserved.
• B
  Only the momentum component in the vertical direction is conserved.
• C
  Only the momentum component parallel to the track is conserved.
• D
  No components are conserved.