A Dog Owner Is Trying to Train His Dog to Respond

A dog owner is trying to train his dog to respond to hand signals. He finds that when he speaks to the dog in a loud, firm voice while making the signal the training goes well. In contrast, if his voice becomes quiet and gentle the dog training doesn't seem to be as effective. This is an example of

A) poor reinforcement scheduling.
B) the importance of the intensity of the UCS.
C) habituation to the quieter voice.
D) short-term habituation to a new UCS.
E) the importance of timing in presentation of the UCS and CS.