Consider a Market with One Large Firm and Many Small

Consider a market with one large firm and many small firms. The supply function of all of the small firms together is S(p) = 200 + p, the market demand curve is D(p) = 400 - p, and the cost function for the large firm is C(y) = 20y. The residual demand curve for the large firm, where D_L is the large firm's demand and y_L is the large firm's output, is

A) D_L(p) = 400 - 21y_L.
B) D_L(p) = 200 - 2p.
C) D_L(p) = 600 - 2p.
D) D_L(y_L) = 200 - 2p - 20y_L.
E) D_L(y_L) = 200 + p + 20y_L.