After obtaining an understanding of a client's controls, an auditor may decide to omit tests of the controls. Which of the following is not an appropriate reason to omit tests of controls?

a. The controls appear adequate.

b. The controls duplicate other controls.

c. Reportable conditions preclude assessing control risk below the maximum.

d. The effort to test controls exceeds the effort saved by not performing substantive tests.

Choose the correct answer.