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 + 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.