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C is corrent because, when considering accounting estimates, an auditor is concerned with whether the estimates are reasonable, as well as with whether all estimates relating to material matters have been developed and the estimates are presented in conformity with GAAP.
A is incorrect because the amount of receivables more than 90% past due is an objective number, not an estimate.
B is incorrect because estimates are used when the outcome of future events related to the estimated item is unknown.
D is incorrect because accounting estimates are often included in the financial statements because relevant data cannot be accumulated on a timely, cost-effective basis.