阅读下面材料, 从A、B、C、D四个选项中选出最佳选项。

    Jumping rope(绳子)is a sport any student can learn to do．You can jump alone or with a friend．You can also help turn a long rope for many others to jump over．If you jump alone, you will need a short rope which should be just long enough for you．

    A rope should be turned slowly．It must also be turned high enough to let you jump safely．Turning the rope safely needs careful timing．In rope jumping, timing means turning the rope slowly enough so that you can jump over it at the right time without its hitting your feet．

  You will notice that when you jump rope, you breathe(呼吸)faster and your face has a healthy colour．But, if you jump too many times at first, you will get too tired．You must learn when to stop for rest(休息)before you jump again．As you practice, you can increase the number(次数)of times you jump without resting．

配方法解方程x^₂-4x+2=0,下列配方正确的是（   ）                     

2014年2月27日，十二届全国人大常委会决定将9月3日确定为一重大历史事件纪念日 。这重大历史事件是(     )