单项选择题 A user reports that their PC is making a strange noise. Which of the following is the cause of the problem?（）

单项选择题 Which of the following is the BEST tool to troubleshoot a DB-9 port?（）

单项选择题 A laser printer displays ‘perform user maintenance’. The manufacturer’s information was not available when the user performed the maintenance, and the laser printer continues to display the maintenance code. Which of the following items needs to be performed?（）