'C', Receiving a "thank you" from somebody after doing something nice is an example of reinforcement. Getting a reward, in this case a thank you, reinforces good behavior. It gives a person to keep doing the good behavior. Punishing in the case of not leaving a tip or not getting employment will punish bad behavior but do not encourage good behavior. An 'A' for not doing the work, does not reward good behavior, it rewards bad behavior.
