It does, but the time interval for hitting the c is very small. I suggest you have your left hand on keyboard for the choices, and your right hand on the c, ready to smack it after your response.

Ohhh, it has? (the icon is not shown in this game, as far as I can see). I will try it!
Thx a lot!

Its a keyboard control (no icon)
The games that use the 'C' to compare answers are EQ Knight, Dr. Compressor, DB King, Filter Expert, Delay Control, Distorted Reality & Reverb Wizard.
It can be used immediately after selecting the answer (right or wrong). You have to be quick, as you only have a few seconds to press C before the next question one comes up.
Probably the feature with the least discoverability.

There is also Practice Mode as well, although I find it less useful than just palying the game and using the compare answer feature for Delay Control. Most games have a pratice mode, however this is only for paid (Pro) memberships