William has been working on a problem with a computer at his company for several hours and still can't ascertain the problem. The computer will not turn on, and William has tried everything he can think of to fix the computer. He has replaced the power cord, the power supply, and even the power strip the computer is plugged into. William decides that it must be the motherboard and takes the computer to his shop to replace the motherboard on the computer. When he plugs the computer in, it turns on. The problem was not the computer—someone had turned off the switch for the wall outlet that the power strip was plugged into.

Where in the troubleshooting process could William have discovered this problem? (Select all that apply.)
Identify the problem.
Test the theory to determine cause.
Establish a theory of probable cause.