I think Tom, as the head of a big department, should either study regularly or ________ his job. [ ]  

A. quits  

B. to quit  

C. quitting  

D. quit

设函数y=f（x）满足f（2+x）=f（2-x），又f（x）在[2，+∞）是减函数，且f（a）≥f（0），则实数a的取值范围是（　　）

A．a≥2

B．a＜0

C．0≤a≤4

D．a＜0或a≥4