Answer;
They became rich by building monopolies
Explanation;
-America's tycoons in the 19th and early 20th centuries, nicknamed as "robber barons," built massive empires and accumulated unprecedented wealth.
-Many of these men gained their vast fortunes either at the expense of their factory workers or by methods that were considered unscrupulous even back then, a time when insider trading wasn't yet outlawed.
-However, some of them also gave away their fortunes to build universities, hospitals, libraries, and museums that still dot America today.