鼠疫在世界范围内难以彻底消灭的最主要的原因为（）

A.鼠疫自然疫源地的存在

B.非洲、亚洲第三世界国家对传染病的监管力度不够

C.家畜亦可能成为传染源

D.某些国家仍保存鼠疫杆菌

E.狩猎者的存在

At the required fire drill conducted aboard a ship，all persons must report to their stations and demonstrate their ability to perform the duties assigned to them（）.

A.by the toolpusher

B.in the station bill

C.by the person conducting the drill

D.at the previous safety meeting