Refer to the Following Table Showing a Monopolist's Demand Schedule

Multiple Choice

Refer to the following table showing a monopolist's demand schedule:
$$\begin{array} { c c } \text { Price } & \text { Quantity } \\\hline \$ 50 & \mathbf { 3 0 0 } \\40 & \mathbf { 6 0 0 } \\20 & \mathbf { 8 0 0 } \\10 & 1,000\end{array}$$
-What is marginal revenue for a price decrease from $50 to $40?

A) $9,000
B) $24,000
C) $30
D) $20
E) $40