On Your Cell Phone,you Have One Ring Tone for People

Multiple Choice

On your cell phone,you have one ring tone for people you want to speak to,one ring tone for people you do not want to speak to,and another ring tone for calls from strangers.In fact,anyone can clearly observe that you show different telephone answering behaviors in response to these different ring tones.Thus,these ring tones serve as

A) primary reinforcers.
B) cognitive maps.
C) unconditoned stimuli.
D) discriminative stimuli.