When implementing a solution to a problem， which of the following is the BEST course of action to take？（）

• A、 Replace all components simultaneously to ensure that the system is functional.
• B、 Replace all hardware associated with the problem first to eliminate the possibility of hardware and move onto software.
• C、 Purchase new parts for the system to confirm that they function.
• D、 Implement one change at a time， reversing the change if it has not resolved the problem.