The most important economic effect of the Internal combustion engine was

they did

Explanation:

the term manifest destiny means "was a widely held cultural belief in the 19th-century United States that American settlers were destined to expand across North America."

Mountain men went to the west to make a profit of the wilderness. They were instrumental in setting up land west of the Mississippi for settlers to move. they contributed to the expansion of the west because they opened up the Oregon train and guided settlers upon it.