McRobert's TV Shop Sells 400 Sets Per Month, at a Price

Multiple Choice

McRobert's TV Shop sells 400 sets per month, at a price of $600 per unit. Market research indicates that the shop can sell one additional set for each $1 it reduces the price. The total revenue (in dollars) is given by , where x is the number of additional sets beyond the original 400. What is the marginal revenue if 300 more units are sold? ​

A) 0$1,600
B) ​-$100
C) 0$400
D) -$400
E) 0$800