某体校准备在甲乙两位射箭爱好者中选出一人参加集训,两人各射了5箭,他们的总成绩（单位：环）相同,小宇根据他们的成绩绘制了尚不完整的统计图表,并计算了甲成绩的平均数和方差（见小宇的作业）．

甲、乙两人射箭成绩统计表

（1）a=　_________　,=　_________　；
（2）请完成图中表示乙成绩变化情况的折线；
（3）①观察图,可看出　_________　的成绩比较稳定（填“甲”或“乙”）．参照小宇的计算方法,计算乙成绩的方差,并验证你的判断．
②请你从平均数和方差的角度分析,谁将被选中．

    Big Ben is not the name of a man. It's the name of a big clock in London. This clock has four faces. So, no matter(无论)where you stand, you can read the time on the face of Big Ben. Each face is the size of a bus. The hands are about four metres long. If you go to London, you may want to visit the Houses of Parliament(英国国会大厦). In that place you will find Big Ben sits at the top of the clock tower in the Houses of Parliament. You will probably hear it and see it. The big clock makes such a loud noise. It goes every quarter of an hour. Big Ben has been the most famous symbol(标志)of  London