50Hz交流电干扰
参考答案：
当排列通过高压输电线时，在地震记录上会出现50周交流电干扰，其特点是频率稳定，一般都在50±3Hz.
The young dancers looked so charming in their beautiful clothes that we took ____pictures of them.
A．many of
B．a large amount of
C．a plenty of
D．masses of
若，则k的值为( )．
A．1
B．1或-2
C．-1或2
D．-2
E．2
