According to historians and specialists in demography,there are（）great population movement(s)in the history of the United States.

A.Britain and the Netherlands　
B.Austria-Hungary and Russia
C.Japan and China
D.Mexico and Cuba

A.famine　
B.English oppression
C.war　
D.Both A&B

A.four　
B.two
C.three
D.None of the above