Which of the following statements about internal control is correct?

a. A properly maintained internal control system reasonably ensures that collusion among employees cannot occur.

b. The establishment and maintenance of internal control is an important responsibility of the internal auditor.

c. An exceptionally strong internal control system is enough for the auditor to eliminate substantive procedures on a significant account balance.

d. The cost-benefit relationship is a primary criterion that should be considered in designing an internal control system.