Why was the late-nineteenth-century southwestern frontier largely Span

Why was the late-nineteenth-century southwestern frontier largely Spanish American?

a. The heavy Spanish influence there was due to the original Spanish settlers.
b. There were very few Anglo Americans who wanted to settle the southwest.
c. It was originally part of Mexico, and many Mexicans still had communities there.
d. Mexico did not allow people from the eastern United States to settle there.
e. English settlers were reluctant to learn Spanish, so they settled elsewhere.

ANSWER
A