A house may serve many purposes, but its main __________ is to provide shelter. [ ]

A. rank      

B. approach

C. function  

D. theme

防疫部门免费为人们接种预防“甲型H1N1流感”的疫苗的目的是（　　）

A．进行非特异性免疫

B．控制传染源

C．切断传播途径

D．保护易感人群