Loss of grasses left topsoil loose and unprotected.
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high winds frequently blew on the great plains.
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valuable topsoil needed by farmers was thus blown away.
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The logical continuation of this chain would be:

Millions of acres of productive farmland became barren desert.

Without grass, the land had no protection from he winds, which blew away the fertile topsoil. And resultedly, we receive a deserted area, with no plants growing there.

d is the best answer for this question.