The answer is D. The question asks how different cultures influenced each other so that rules out options A & B. Option C is about the environment being destroyed, and that really has nothing to do with foreign influence.
One simple and easy way that scientists can accurately date fossils found is through the ground levels. When you take a big chunk out of the earth, you can see the changes in the texture in the "ground" Each "layer" represents a set of years. And as you get further and further into the ground, fossils become older and older.
The correct answer is C: the Mackenzie River!
