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the surface area of the inner membrane,
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Read the sentence from the passage.

"Then Europeans arrived and began to move westward, and some Native American trails became migration routes."

Why is the event important to the plot?
A.
It clarifies the relationship between the groups of people who wanted to use the trails.
B.
It explains how the earliest trails were used by too many people for trade.
C.
It describes what the trails looked like and how difficult they were to travel on.
D.
It shows how the purpose of the trails changed as people started to move west.