When you connect an external monitor to your portable computer， the same image appears on both monitors.  You need to drag a window from one monitor to the other.  What should you do？（）

• A、Change the display setting to extend the desktop.
• B、In the Microsoft Windows Mobility Center window， turn on the Presentation Settings feature. 
• C、Configure the external monitor as the primary display and the built-in monitor as the secondary display.
• D、Change the secondary display resolution settings to make them equal to or less than the primary display resolution settings.