Which two statements are true about mouse button variables?（）

A、SYSTEM.MOUSE_BUTTON_PRESSED returns a string. 

B、SYSTEM.MOUSE_BUTTON_PRESSED returns a numeric value. 1Z0-132 World Leaders In Certifications Material - Test-king.com 

C、A possible value for SYSTEM.MOUSE_BUTTON_MODIFIERS is Control+Alt+Delete. 

D、SYSTEM.MOUSE_BUTTON_MODIFIERS can only be references in form-level triggers. 

E、SYSTEM.MOUSE_BUTTON_MODIFIERS identifies which special key was pressed in conjunction with the usual mouse button action.