题目

The surprising experiment I am about to describe proves that air is all around you and that it pressed down upon you. Air pressure is a powerful force. When you swim underwater, you can feel water pushing on your body. The air all around you does the same. However, your body is so used to it that you do not notice this. The pressure is caused by a layer of air called the atmosphere. This layer surrounds the earth, extending to about five kilometers above the earth’s surface.  The following experiment is an easy one that you can do it at home. But make sure that you are supervised because yo如图所示的电路，电源电压恒定不变，闭合电键S发现小灯L不亮，若电路中只有一处故障，且只发生在灯L或电阻R上，为检测故障所在，下列做法和判断正确的是A.用电压表并在灯L两端，电压表无示数，则灯L一定短路B.用电压表并在电阻R两端，电压表无示数，则电阻R可能短路C.用一个检测小灯L'并联在电阻R两端，若灯L'亮，则灯L一定短路D.用一个检测小灯L'并联在电阻R两端，若灯L'不亮，则灯L一定断路