How is it possible for a U.S. firm to have an effective tax rate that is less than the U.S. federal statutory tax rate?

a. The firm has expenses that are not deductible for tax purposes.

b. Tax rates in foreign countries where the firm operates are higher.

c. Tax rates in foreign countries where the firm operates are lower.

d. It is not possible for a firm to have an effective tax rate different from the U.S. federal statutory tax rate.