The above figure shows Bob's utility function, which is

The opportunity cost of an action:

a. is equal to the marginal cost of an action b. is equal to explicit cost c. is equal to the cost of the next best alternative forgone d. is the total cost of an action

If a household has $40,000 in taxable income and its tax liability is $20,000, the household's average tax rate is

a. 10 percent. b. 25 percent. c. 40 percent. d. 50 percent