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. What is the average speed of the rail car $+$ bowling balls system some time after the collision?

• A
  $(M + Nm)v_0/M$
• B
  $Mv_0/(Nm + M)$
• C
  $Nmv_0/M$
• D
  The speed cannot be determined because there is not enough information