A Store Has Determined That the Weekly Sales of a Product

A store has determined that the weekly sales of a product is related to the number of customers who visit the store and the square feet of shelf space, x, according to the following equation: -20x² - 10C² + 40Cx + 120x - 200C + 600. C represents the hundreds of customers who visit their store. If a store averages 200 customers per week, which of these methods represents the simplest way to determine the optimal number of square feet of shelf space for the product?

A) Lagrangian functions
B) Excel Solver
C) substitution method
D) solution of simultaneous equations
E) none of the above