I am almost certain I had a question like this before on a test. I believe the answer is somewhere along the lines of, "someone who moves around a lot, following animals". I hope this somewhat helps :).
Answer: Mexico's claim to Texas stood in the way of American expansion to the Pacific Ocean
Explanation: Took the test
Answer:
the 4th option
Explanation:
I could be wrong about this, but that is my best guess.
The answer is The Panama Canal :)