Fill in the blanks to make the following statements correct.
a. In the long run, if a firm is maximizing its profits while producing a given level of output, then this firm is also ______ its costs.
b. In the long run, all factors of production are ______. There are no ______ factors.
c. Profit-maximizing firms employ factors of production such that the marginal products per dollar spent on each factor are ______.
d. Firms adjust their methods of production in response to changes in relative prices. This is known as the ______.