Answer: D
Explanation: Hi there.
I will explain why the others are wrong and d is right.
A. Using melted snow is a good idea to water the crops, but it does not help with the rocky and slanted terrain.
B. This would be impossible, because the Andeans did not have the necessary technology.
C. Thick-rooted crops can help them stay on the mountain top, but it is very likely that erosion will weaken the slanted terrain it grows on.
D. is right because the land had to be flat in order for the farmers to grow crops.