为防止电器设备绝缘损坏而使人体有遭受触电危险，将电器设备的金属外壳与（）连接称为保护接零。
参考答案：零线
We bought _____ a present, but _____ didn't like it. [ ]
A. they ; them
B. them; they
C. themselves; their
D. theirs; they
简述压刨床前、后压紧器的作用。
