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D is corrent because the auditor should investigate fluctuations that are not expected. The investigation would ordinarily begin with inquiries of management. If management is unable to provide an acceptable explanation, the auditor should perform additional procedures to investigate the fluctuation further.
A is incorrect because management has not restricted the scope of the audit; management simply cannot explain the fluctuations.
B is incorrect because no information is given which would indicate the need for withdrawal from the engagement.
C is incorrect because management fraud is only one of many possible explanations. Thus, an intensified examination would not be specifically planned to detect management fraud.