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

骨骼中铅的主要存在形式为（）

A.氧化铅

B.可溶性磷酸氢铅

C.不溶性磷酸氢铅

D.可溶性磷酸铅

E.不溶性磷酸铅