An isolated rail car originally moving with speed $v_0$ on a straight,frictionless,level track contains a large amount of sand. $A$ release valve on the bottom of the car malfunctions,and sand begins to pour out straight down relative to the rail car. Is momentum conserved in this process?

  • A
    The momentum of the rail car alone is conserved.
  • B
    The momentum of the rail car $+$ sand remaining within the car is conserved.
  • C
    The momentum of the rail car $+$ all of the sand,both inside and outside the rail car,is conserved.
  • D
    Both $(A)$ and $(C)$.

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