A lash adjuster keeps pressure constant on engine valves, thereby increasing fuel efficiency in automobile engines. The relationship between price $()$ and monthly demand $(D)$ for lash adjusters made by the Wicks Company is given by this equation: $D=\frac{1,800-p}{0\mathrm{.}10}.$ What is the demand $(widehat(D))$ when total revenue is maximized? What important data are needed if maximum profit is desired?

The total revenue is maximized when demand is $9000$ units per month. $($Round to the nearest whole number.$)$

Select all important data that are needed if maximum profit is desired.

A$.$ Variable cost per lash adjuster.

$($B$.$ Variable cost per engine valve.

Variable cost per gallon of fuel.

D$.$ Fixed cost per month.