我国对传染病分为甲类、乙类和丙类进行管理，是根据（）

A.发病率的高低

B.传染性大小和危害性

C.是否能得到有效的治疗

D.根据感染后免疫力是否持久

E.是否具有有效的疫苗预防

Though difficult, I ________ him find a job, which made him very pleased.

A．tried to help

B．managed to help

C．tried helping

D．succeeded to help