题目

His sister _____ her hometown for three years . She will return next year .    A. left    B. has left    C. has been away    D. has been away from 答案：D如图所示，在倾角为θ的斜面上，放一质量为m的光滑小球，小球被竖直挡板挡住，则球对挡板的压力为（ ）A.mgcosθ B.mgtanθ C.mg/cosθ D.mg