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Which of the following sentences from the text best supports Turner's
purpose in describing the significance of the frontier in American history?
A. Most celebrated of these conferences was the Albany congress of
1754, called to treat with the Six Nations, and to consider plans of
union.
B. The tax-gatherer is viewed as a representative of oppression.
C. The recent Populist agitation is a case in point.
D. The existence of an area of free land, its continuous recession,
and the advance of American settlement westward, explain
American development.