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C is corrent because machine operators should not be supervised by the programmer. Good internal control in a computer system requires that operators, programmers, and the library function be segregated.
A is incorrect because machine operators should (by nature of operating the system) have access to error messages and will distribute them to the control group.
B is incorrect because storing backup files off-premises will improve internal control. Reconstruction of files, if necessary, will be possible.
D is incorrect because machine operators should not have access to the systems manual. Operators should not have complete information on the operation (and weaknesses) of the overall system.