_______ his help, I finished the job in time.
Don't worry about the ____ (safe) of the boy, he'll be all right.
人之所以能在有大量病原体存在的环境中健康生活,是因为人类在漫长的进化过程中,形成了防御病原体的三道防线,下列物质或结构属于第二道防线的是( )