A Firm Uses a Single Input to Produce Its Output

A firm uses a single input to produce its output, which is sold in a competitive market.It gets quantity discounts on purchases of its input.If it buys x units of the input, the price it must pay per unit of input is 400/x+ 4.If it buys no inputs, it doesn't have to pay anything.The firm's production function is f(x) =40x - x².If the price of the firm's output is 1, the profit-maximizing amount of input to buy is

A) 0.
B) 36.
C) 18.
D) 27.
E) None of the above.