题干
. Everyone must have known the death of the waitress，________?
A：must they
B：mustn't they
C：haven't they
D：have they
答案(点此获取答案解析）
C
A：must they
B：mustn't they
C：haven't they
D：have they
、 
、 
表示的是电流表或电压表，当开关S1、S2都闭合时，灯正常发光，各电表均正常工作．下列说法正确的是（ ） 
