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 $(C)$.