B is corrent because even though the computer system is fully protected, unscheduled maintenance will require a certain amount of downtime.
A is incorrect because a fully protected computer system has adequate internal controls which would provide for keypunching errors. Downtime would not arise from this type of error.
C is incorrect because a fully protected computer system has alternative power sources which would provide for electrical power losses and, therefore, downtime would not exist for this reason.
D is incorrect because an unauthorized entry would be thwarted in a fully protected system. Thus, downtime would not arise from unauthorized entries.
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