Which of the following is TRUE of the price charged by a monopolistically competitive firm at the profit-maximizing level of output?

The following graph is the production possibilities curve of a nation:

Refer to the above graph. The marginal opportunity cost of the fourth unit of bread is:

A. 0 unit of drill presses
B. 1 unit of drill presses
C. 3 units of drill presses
D. 4 units of drill presses

When the marginal costs of adding workers to a firm are increasing, that firm is experiencing ______.

a. constant returns to scale b. average variable cost c. declining marginal utility d. diminishing returns of labor