锅炉定期排污前，应适当保持低水位，且不可两点同时排放，以防低水位事故发生。
参考答案：错
During the class, Tom slipped out of the classroom ____the back door so as not to be found by the teacher.
A．across
B．in
C．by
D．beside
35kV变电所宜设置不低于( )高的实体围墙。城网变电所、工业企业变电所围墙的高度及形式，应与周围环境相协调。
A．1.2m
B．2.2m
C．3.2m
D．4.2m
