理想电流源输出恒定的电流，其输出端电压（）。
A、恒定不变
B、等于零
C、由外电阻决定
参考答案：C
The film _____ for ten minutes when I got to the cinema . [ ]
A. had begun
B. has been on
C. had been on
D. has begun
某水利工程受弯构件，受压区钢筋采用绑扎接头，接头面积占总受力面积( )
A．25% B．30% C．50% D．70%
