生物体生命活动的基本结构和功能单位是（　　）

A．细胞

B．细胞膜

C．细胞核

D．线粒体

—Are you free after school?

—Sorry.I've planned to treat a friend of mine to dinner________for his help.[ ]

A. in addition  

B. in turn

C. in return  

D. in total