Fill in the blanks to make the following statements correct.
a. Marginal revenue product is calculated by multiplying ________ and ________.
b. Marginal revenue product represents the addition to a firm's total revenue as a result of .
c. To maximize its profits, a firm will hire all units of a factor that have an ________ greater than or equal to its ________.
d. To maximize its profits, any firm will hire each factor of production to the point where the factor's ________ equals the ________.