momentum question from 2018 as physics

The horizontal runway in Figure 2 has negligible friction and air resistance. As the
truck moves along the runway, it starts to rain. The rain falls vertically and water
collects in the truck.
Discuss whether there are any changes in the momentum of the truck and collected water.

My answer was '
momentum is conserved in the collision between the truck and rain. So momentum of truck before collision + momentum of rain before collision as it hits truck = momentum of truck with water collected. But the momentum of rain is 0 after it hits truck (it gets collected) and because momentum in system has to be conserved, the heavier truck must gain momentum'.

But the correct answer was the momentum of the truck remains the same so its velocity decreases in order to compensate for the greater mass.

Why is my answer wrong? Shouldn't we count the momentum of the rain before hand too because it's included in the system?

The horizontal runway in Figure 2 has negligible friction and air resistance. As the
truck moves along the runway, it starts to rain. The rain falls vertically and water
collects in the truck.
Discuss whether there are any changes in the momentum of the truck and collected water.

My answer was '
momentum is conserved in the collision between the truck and rain. So momentum of truck before collision + momentum of rain before collision as it hits truck = momentum of truck with water collected. But the momentum of rain is 0 after it hits truck (it gets collected) and because momentum in system has to be conserved, the heavier truck must gain momentum'.

But the correct answer was the momentum of the truck remains the same so its velocity decreases in order to compensate for the greater mass.

Why is my answer wrong? Shouldn't we count the momentum of the rain before hand too because it's included in the system?

This question stirred up some great discussions among the students in this thread and you may be interested. https://www.thestudentroom.co.uk/showthread.php?t=5239184&page=16

Hey so I just came across your question, although I dont know which board you're having, is it cambridge? because mine is edexcel and we just did momentum in A2. So I'm having good a good guess at this, and I hope my answer satisfies your question. Notice that it's specifically mentioning that the rain falling was vertical. As momentum is a vector quantity, this rain has no horizontal component in the direction of motion of the truck. The rain is making no contribution to the total momentum now. So the total momentum of the truck before and after should remain the same according to the law of conservation of momentum, but now the total mass has increased due the rain, so to keep the p constant, v must decrease. As p=mv.
Someone else may correct me if I'm wrong