Dollars are called greenbacks because that is the color of the back side of the paper money.

美元之所以又叫“绿背”（美钞），是因为美钞的背面是绿色的。

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Returns the system color used for the back side of a view while it is being flipped.

返回一个系统颜色当视图被点了一下之后的背景部分的颜色。

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Returns the system color used for the back side of a view while it is being flipped.

返回一个系统颜色当视图被点了一下之后的背景部分的颜色。

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