Answer:
i just had a test on this with 2 attempts and got it wrong on both but i do know that the answer is not...
the steepness of the models hillside
or
the rate of erosion on the hills
so the answer is either...
the amount of water sprayed on the hills
or
the type of material used to construct the model hills
but i think its the amount of water sprayed on the hills...i hope i helped a lil ;)
Explanation: