---     Bob doing homework?

---No, he isn’t. He is        a letter.

A．Does, writes

B．Is, writes

C．Does, writing

D．Is, writing

内燃式热风炉燃烧时，如出现顶温烧不上去而烟道温度却上升很快，烟气分析中CO，CO₂含量均高，此时可认为（）已被烧穿，发生气流短路。