He function c(x) = 400x – 0.2x2 represents the total costs for a company to produce a product, where c is the total cost in dollars and x is the number of units sold. which statement is true?

To determine the maxima and minima of the polynomial, differentiate the given based on x and equate to 0.
                                   C(x) = 400x - 0.2x²
                               dC(x) / dt = 400 - 0.4 x = 0 
The value of x is 1000. This is the value of the maxima. As the value of C(x) continously becomes lesser as the value of x is set higher, the minima is not identified. Substitute x to the original equation,
                               C(x) = (400)(1000) - 0.2(1000²) = $ 200,000
Thus, the answer is letter B. 

What mathematical statement is missing ? ? the answer is b