A River Emerges from a Valley onto a Flat Plain,but

A river emerges from a valley onto a flat plain,but the river itself lies in a narrow,shallow canyon as it crosses the plain,then plunges into a steep sided canyon as it flow past the plain.A geologist floating down the canyon headed for the whitewater in the canyon notices the river is flowing through lake deposits as it flows across the plain.What happened in this area?

A) The river is at base level throughout its trace and has been for a long time.
B) The river used to flow into a lake,which formed a temporary base level,but in recent geologic time the lakes dam broke,and the river is now downcutting in response to the change in local base level.
C) The river has cut a deep canyon that crossed through a lake,and this process has been continuous for a long time.
D) The geologist can't be right; this is an impossible scenario.

Correct Answer:

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