二端网络内若包含有（）则称为有源二端网络。
A.电流
B.电压
C.电源
D.电动力
参考答案：C
根据文件的逻辑结构，文件可以分为( )两类。
A．字符串文件和页面文件
B．记录式文件和流式文件
C．索引文件和串联文件
D．顺序文件和索引文件
As soon as the children _____ themselves in their seats in the theater, the curtain went up. [ ]
A. sat
B. settled
C. took
D. put
