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.
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.