Solve the Problem $p$ (In Dollars) and the Quantity $x$ Sold of a Certain Product Obey the Demand Equation

Multiple Choice

Solve the problem.
-The price $p$ (in dollars) and the quantity $x$ sold of a certain product obey the demand equation $x = - 6 p + 120 , \quad 0 \leq p \leq 20$ .
What quantity $x$ maximizes revenue? What is the maximum revenue?

A) $60 ; \$ 600$
B) 90; $\$ 450$
C) $30 ; \$ 450$
D) $120 ; \$ 600$