静电接地安装管道系统的对地电阻值超过100Ω时，应设（）处接地引线，接地引线宜采用焊接形式。
A．1
B．2
C．3
D．4
参考答案：B
When I last saw them, the police had chased the robbers down Columbus Street.
A．were chasing
B．was chasing
C．chased
D．were on a chase
