After several attempts to distract his young parrot from chewing on furniture, George reluctantly took an expert' s advice and gently hit the parrot' s beak whenever the bird started to chew furniture. The bird stopped chewing furniture, but it is now afraid of hands and will sometimes bite. Since chewing on the furniture would not have hurt the bird, George should not have hit it. When Carla' s puppy escaped from her yard, it bounded into a busy street. Although Carla does not generally approve of physical discipline, she hit the puppy sharply with her hand. Now the puppy enters the street only when accompanied by Carla, so Carla was justified in disciplining the puppy. Which one of the following principles, if established, would justify the judgments about George's and Carla's actions? ['When disciplining an animal physically, a trainer should use an object such as a rolled up newspaper to avoid making the animal frightened of hands.', 'Physical discipline should not be used by an animal trainer except to correct persistent behavior problems.', 'When training an animal, physical discipline should be used only when such discipline is necessary to correct behavior that could result in serious harm to the animal.', 'Using physical discipline to train an animal is justified only when all alternative strategies for correcting undesirable behavior have failed.']