Using just the Ctrl or Alt keys as a trigger

Hello everyone! I have an issue with using the Ctrl and Alt keys as "when user presses a key" triggers...

In my project, I want to create a trigger when the learner presses just the Ctrl key, not "Ctrl+ some other key"

Same scenario for "Alt" (just Alt, not Alt+something else)

When I press either key while building the trigger it appears in the box, but will not "stick" as it is looking for an additional key to be pressed in tandem with either the Ctrl or Alt - and I just want the learner to press the single Ctrl or Alt key.

Is this possible? ...I really hope so!  

Thanks a million, I hope you all have a great day!

-Tom