装饰用的小彩灯每个两端电压不允许超过10V,问至少要串联多少只灯,才能用家庭电路做供电电源.( )
A.20只
B.21只
C.22只
D.23只
答案:C
There is________ that we will be successful.[ ]
A. doubt
B. no doubt
C. refer to
D. take up
概要设计