(2005•乌兰察布)如图:把△ABC沿AB边平移到△A′B′C′的位置,它们的重叠部分(即图中阴影部分)的面积是△ABC面积的一半,若AB=
,则此三角形移动的距离AA′是( )

A.
-1
B.
C.1
D.
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A.
B.
C.1
D.