Not only his parents but also his brother _____ from here for two days. They haven’t been back.

A. have left

B. has left

C. has been away

如图，小明家新买的房子准备装修，为了将水泥从地面送上6m高的楼顶，他在楼上安装了一个滑轮组，所用的滑轮每只2kg，每次只提升一袋水泥，每袋水泥的质量为50kg．（不计绳重和摩擦．g取10N/kg）．求：

（1）每袋水泥受到的重力大小；

（2）每次做的有用功；

（3）滑轮组的机械效率．