Question

why plains and river valleys are densely populated areas in the world
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Answer 1

Explanation:

plains and river valleys are densely populated because the rivers nearby provided water.The construction and development can be more in plains and river valleys

Answer 2

Rivers and valleys are densely populated as these areas are perfectly suitable for agriculture. Plains provide fertile soil and is suitable for constructions and large buildings. Roads can be made on these plains so that small to big vehicles can run over them providing easy transportation facilities to people