半径为R的大圆盘以角速度ω旋转，如图所示，有人站在盘边P点上随盘转动，他想用枪击中在圆盘中心的目标O，若子弹的速度为v₀，则（    ）

假定英语课上老师要求同桌之间交换修改作文，请你修改你同桌写的以下作文。文中共有10处语言错误，每句中最多有两处。每处错误仅涉及一个单词的增加、删除或修改。

增加：在缺词处加一个漏字符号（∧），并在其下面写出该加的词。

删除：把多余的词用斜线（\）划掉。

修改：在错的词下面画一横线，并在该词下面写出修改后的词。

注意：1). 每处错误及其修改均仅限一词；

      2). 只允许修改10处，多者（从第11处起）不计分。

    The other day, some of my classmates and I took a bicycle trip along the "Huan Shan Green Road", that was specially built for people to relax ourselves. The scenery along the road was fascinating, with trees, flowers, hills and lakes on both side. We stopped by a lake for a rest, where a great many of people are playing happily. But something unpleasant caught our eyes. There was rubbish here or there, and there were many plastic bags and bottles floated on the surface of the lake. Such beautiful place was so serious polluted. What a shame! In the end, we couldn't help collecting the rubbish after we left.

在西欧，“小麦的价格从十六世纪初到世纪中未有上升，而在世纪末却涨到了四倍于世纪中的价格”。出现这一现象的主要原因是（  ）