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B is corrent because a check digit, while normally at the end of an account number, may be placed consistently in any position in the account when adequate computer programming exists (i.e., the mathematical calculation of the check digit can be performed regardless of placement).
A is incorrect because placing it at in the middle does not increase internal control as compared to other places. 
C is incorrect because placing it at the end does not increase internal control as compared to other places. 
D is incorrect because placing it at the beginning does not increase internal control as compared to other places.