They left rich soil when flood waters receded. So the correct option for the given question is option "d". It has been seen that rivers carry with them very rich and fertile soil which the farmers prefer because the amount of crop and the quality of crop is much better than they can get in any other place. The civilization of ancient Egypt as well as the Indus valley civilization were totally dependent on the Tigris and the Indus river respectively, as far as irrigation was concerned. Seasonal flooding always brings with it new fertile soil for irrigation.
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Driven in part by economic concerns, and in part by frustration with the straitened social conditions of the South, in the 1870s African Americans began moving North and West in great numbers. In the 1890s, the number of African Americans moving to the Northeast and the Midwest was double that of the previous decade.
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