Exercise No2 - Answer

Not so easy. Dummy may ask declarer about a possible revoke and though he doesn't do so, the effect is the same in a) and in b): declarer becomes aware of his revoke. Therefore he should be allowed to change the card he played. L61B.

In c) declarer did not revoke so ©3 is a played card.

In case a) North certainly should get a reprimand.