Regarding the internal audit function, which of the following statements is true?

a.Since many internal auditors have accounting backgrounds, the internal audit function should ideally be included within a company's accounting subsystem

b.It is not proper for internal auditors to perform a fraud investigation within any part of their company's system

c.Because of the independence of external auditors, they should never accept previous work of evaluating controls performed by a company's internal auditors

d.Within a company's system, it is preferable to establish the internal audit function as a separate subsystem