—What do you think of teaching, Bob?

—I find it fun and challenging. It is a job ______ you are doing something serious but interesting.

A．where

B．which

C．when

D．that

搞好个人卫生和环境卫生，消灭媒介生物，清除中间寄生生物和消毒工作是为了（　　）

A．控制传染源

B．保护易感人群

C．切断传播途径

D．消除病原体