调查人群流脑血凝抗体水平的对象为（）

A.自然人群（包括以往有过该传染病史或有过预防接种史者）

B.健康人群（除去有预防接种史与传染病史者）

C.易感人群

D.传染病的接触人群

E.特殊人群

The straight stream capability of an all-purpose nozzle is used in fighting a class A fire to（）.

A.shield fire fighters from radiant heat

B.break up burning material

C.get the most water possible on the fire

D.drive heat and smoke ahead of the fire fighters