Pls let me understand this q more clearly...
Rather than continue to produce most of the items necessary for subsistence, a growing number of farm families during the first decades of the nineteenth century began to specialize in the production of grain or cotton and to use the cash proceeds from selling their crops for buying necessities.
(A) selling their crops for buying
(B) the sales of their crops for buying
(C) their selling of crops so as to buy
(D) their selling crops for buying of
(E) the sale of their crops to buy
Selling is a verb sale(s) is noun. we need sale or sales and hence A C D are out
to buy is better than for buying. E remains