An unpowered open truck is coasting horizontally with a constant velocity along a frictionless track underneath a grain bin when grain falls vertically into the truck, partly filling it as it moves along. Which of the following will happen to the horizontal momentum of the truck with the accumulating grain?

W) It will decrease

X) It will increase

Y) It will stay the same

Z) It will first increase and then decrease
Y) IT WILL STAY THE SAME