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A is corrent because if controls appear adequate, the auditor tests them unless, (1) the costs of testing are expected to exceed the savings in substantive tests or (2) the controls are redundant to other internal control activities. Therefore, this is not a valid reason for omitting tests of controls.
B is incorrect because when controls duplicate operative controls existing elsewhere in the system, tests of controls will not be required for both sets of controls. Therefore, the fact that the controls are redundant to other controls is a valid reason for omitting testing on them.
C is incorrect because tests of controls may be omitted if it is determined that the costs of testing are expected to exceed the possible savings in substantive tests.
D is incorrect because the auditor should not expend the effort on the tests of controls if the review of the system indicates that there are conditions which would preclude reliance on the system.