In the United States, there is always         flow of people to areas of         country where more jobs can be found.[ ]

A. a; the              

B. the; a              

C. the; the            

D. a; a

最常见偏头痛的是（）

A.无先兆偏头痛

B.偏头痛

C.紧张型头痛

D.低颅压性头痛

E.颅内占位性病变