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C is corrent because the substitution of already counted securities for securities which should be on hand but are not may allow for concealment of material shortages through temporary transfers. The auditor should plan to count securities in all accounts simultaneously in order to safeguard against concealment.
A is incorrect because the auditor will identify the unauthorized event when the count is performed in this circumstance. 
B is incorrect because while this is a concern, another answer addresses the area more completely.  
D is incorrect because while this may be a concern, another answer addresses the area more completely.