During a Routine Audit of a Customer Service Hotline, an Internal

During a routine audit of a customer service hotline, an internal auditor noticed that an unusually high number of customer complaints pertained to payments not being applied to the customers' accounts. Which of the following would most likely be the reason for the high volume of complaints?

A) An ineffective customer service department.
B) Poor controls in the invoice approval processes.
C) Check tampering by an employee.
D) Submission of fraudulent expense reports.