下列（）是保证电气作业安全的组织措施。
A、工作许可制度
B、停电
C、悬挂接地线
参考答案：A
Those flowers were for your mother on her seventieth birthday， but as she is away， I would be glad if you accept them.
A．offered
B．produced
C．meant
D．supplied
设x1和x2是方程2x2-4x+5=0的两个根，则|x1-x2|=（）
A．0
B．
C．
D．不存在
