The Trial-And-Error Method Should NOT Be Used Under Which of the Following

The trial-and-error method should NOT be used under which of the following situations?

A) Whatever change you're about to make can be undone easily.
B) The system the change is being applied to is new,so no chance of data loss.
C) The problem is being discussed over the phone and you're instructing an untrained user.
D) A server or internetworking device you're troubleshooting is currently in use on the
E) twork.