Adjust the potentiometer until you do not hear a hum from the loudspeaker. 调整电位器，直到你听不到从扬声器发出的嗡嗡声。
A.If there is hum from the loudspeaker, don't adjust the potentiometer B.Adjust then potentiometer unless hear a hum from the loudspeaker C.Adjust the potentiometer until the loudspeaker no longer hums.正确答案C
