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
