Refer to the Following $$\begin{array} { l } Q _ { d } = 120 - 10 P \\M C _ { A } = 4 + ( 1 / 5 ) Q _ { A } \\M C _ { B } = 6 + ( 1 / 10 ) Q _ { B }\end{array}$$

Multiple Choice

Refer to the following.
A firm with two plants, A and B, has the following estimated demand and marginal cost functions:
$$\begin{array} { l } Q _ { d } = 120 - 10 P \\M C _ { A } = 4 + ( 1 / 5 ) Q _ { A } \\M C _ { B } = 6 + ( 1 / 10 ) Q _ { B }\end{array}$$
-What is the firm's marginal revenue function?

A) MR = 12- (1/5) P
B) MR = 12 - (1/5) Q
C) MR = 120 - 20P
D) MR = 120 - 20Q
E) none of the above