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A is corrent because the preparation of disbursement vouchers and signing of checks places an individual in a position in which s/he can both prepare erroneous vouchers and then pay them.
B is incorrect because the person requesting the merchandise will be able to determine whether the appropriate merchandise has been received and should, therefore, approve its receipt.
C is incorrect because the person signing the checks should cancel supporting documents to avoid duplicate payment.
D is incorrect because the individual receiving the merchandise should prepare a receiving report so as to establish control over the goods.