题目

如图所示，斜面体固定在水平面上，小物块A与斜面体间接触面光滑．在小物块沿斜面体下滑的过程中，斜面体对小物块的作用力A．垂直于斜面，做功为零B．垂直于斜面，做功不为零C．不垂直于斜面，做功为零D．不垂直于斜面，做功不为零            答案：A 解析:略  (1)I saw him ________ at the back of the classroom． (2)I saw him ________ himself at the back of the classroom． A．sitting B．seated C．being seated D．seating