Which of the following statements is most accurate?

a. ERP systems are commercial software packages that are sometimes called turnkey systems because they can be implemented by the user with little or no modification.

b. Substantive tests provide evidence focused on the system controls.

c. Public accounting firms that provide attest services are not allowed under SOX to offer IT advisory services.

d. Both small and large firms that have standardized information needs are potential customers for commercial software.

e. External auditors may rely on the work of internal auditors only if they report to the controller or the CEO of the client company.