Solve the Problem $\mathrm { M } ( \mathrm { x } )$

Multiple Choice

Solve the problem.
-The number of mosquitoes $\mathrm { M } ( \mathrm { x } )$ , in millions, in a certain area depends on the June rainfall $\mathrm { x }$ , in inches, according to the function $\mathrm { M } ( \mathrm { x } ) = 20 \mathrm { x } - \mathrm { x } ^ { 2 }$ . What rainfall produces the maximum number of mosquitoes?

A) 20 in.
B) 400 in.
C) 0 in.
D) 10 in.