The Number of Mosquitoes $M(x)$ , in Millions, in a Certain Area Depends on the Certain

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 formula $M(x) =9 x-x^{2}$ . What rainfall produces the maximum number of mosquitoes?

A) 0 in.
B) $81 \mathrm{in.}$
C) 4.5 in.
D) 9 in.