The first one, valleys. Rivers had to be built over, no one wanted their farms to be turned into a railroad (or at least those with common sense), and deserts had no water and was really hot during the day and cold at night.
Differing physical forces is the correct answer :)
The lithosphere is less dense
By mid point equation, x = 2, y = 0.5, the answer is 0.5
C is the answer.
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