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 profit-maximizing price?

A) $7
B) $8
C) $9
D) $9.50
E) none of the above