Which of the following statements are correct?
a) Substantive tests of transactions are concerned with monetary misstatements.
b) Dual-purpose tests can improve audit efficiency.
c) Tests of controls are concerned with monetary misstatements.
d) Tests of controls check the operating effectiveness of controls.
e) An audit procedure should either be a test of controls or a substantive test of transactions.
f) Substantive tests of transactions check the operating effectiveness of controls.