The price (in dollars) and the quantity sold of a certain product satisfy the demand equation x=-8p+400. Find a model that expresses the revenue R as a function of the price p, and determine the revenue when the price is $50.

A) R = -8p^2 + 400p
B) R = 8p^2 - 400p
C) R = 400p - 8p^2
D) R = 400p + 8p^2