气压机启动前一定要排凝暖机，停机后一定要（）。
参考答案：排凝
根据《大气污染物综合排放标准》，工业生产尾气确需燃烧排放的，其烟气黑度不得超过林格曼（）。
A.1级
B.2级
C.3级
D.4级
I've become good friends with several of the students in my school _____ I met in the English speech contest
last year. [ ]
A. who
B. where
C. when
D. which
