Why did new mexico, texas, and california attract expansionists in the 1820s?

They had massive oil reserves

Because they were rich in resources and thinly settled.

Explanation:

Back int he early 1800s, many settlers moved to the west. They went over the Appalachian Mountains; to new territories and states. These pioneers moved to the regions of Texas, California, and some other lands that belonged to Mexico. Obviously, the fact that the resources were abundant would only bring more people to those areas.

the tallest building in the western hemisphere and the sixth tallest building in the world

the answer is a, salt plain