The blind man thought___________necessary to pay back the help people gave him when it was sunny.

A．that

B．it

C．this

D．him

人体能对抗再感染的主要原因是 （）

A.非特异性免疫功能

B.特异性免疫功能

C.预防用药

D.注射疫苗

E.增强体质