看图完成对话。

A: What 1.____ you 2.____ 3.____ the tree?

B: I can see some 4.____. 

A: What are they doing?

B: They are 5.____.

烟道尾部发生二次燃烧时，排烟温度会急剧下降，烟道内负压会急剧上升。