建筑的电缆井、管道井、排(气)烟道、垃圾道等竖向井道，井壁上的检查门为（）防火门。

A．甲级 

B．乙级 

C．丙级 

D．丁级

We are taught that a business letter should be written in a formal style ________ in a personal style.[ ]

A. rather than  

B. other than

C. better than 

D. more than