A. Ctrl+Windows
B. Alt
C. Shift
D. All of the Above

Brief Facts About Modifier Keys:

A modifier key is a special key on a computer keyboard that modifies the normal action of another key when the two are pressed in combination. A modifier key works in combination with other keys to do various interesting and unbelievable things. Examples of modifier keys are; Shift, Control, Alt, Command, Option, and Function.