34.With what topic is the passage primarily concerned?
(A) The river systems of portions of North America.
(B) Certain geological features to the North American continent.
(C) An exploratory trip sponsored by the United States government.
(D) The discovery of natural resources in the United States.
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統計: A(0), B(1), C(1), D(0), E(0) #802772
統計: A(0), B(1), C(1), D(0), E(0) #802772