The Number of Mosquitoes M(x), in Millions, in a Certain $M ( x ) = 20 x - x ^ { 2 }$

Multiple Choice

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

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