题目

根据句意及首字母提示完成单词。【小题1】It is well－known that there are four s________ in a year.【小题2】Everyone else in my class was invited e________ me, but I don't know why.【小题3】It's polite to k________ on the door before entering a room.【小题4】Some students have to wear glasses in class because they can't see the b________? clearly.【小题5】Our teacher has given us some advice that is well w________ considering.如图所示，质量为，的物体A经一轻质弹簧与下方地面上的质量为的物体B相连，弹簧的劲度系数为k，A、B都处于静止状态．一条不可伸长的轻绳绕过轻滑轮，一端连物体A，另一端连一轻挂钩．开始时各段绳都处于伸直状态，A上方的一段绳沿竖直方向．现在挂钩上挂一质量为的物体C并从静止状态释放，已知它恰好能使B离开地面但不继续上升．若将C换成另一个质量为()的物体D，仍从上述初始位置由静止状态释放，则这次B刚离地时D的速度的大小是多少？(已知重力加速度为g)．