The Mexican war helped the United States to gain control of new western lands and
A) created a free trade zone with Mexico
B) provided the reservation system for Native Americans
C) lead to the establishment of the Northwest Ordinance
D) establish boundaries with Mexico

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Explanation:

The Mexican War helped the United States to gain control of new western lands and  establish boundaries with Mexico.

D. establish boundaries with Mexico

Explanation:

In addition, the United States took over $ 3.25 million of debt that the Mexican government owed to U.S. citizens