除去下列物质中含有的少量杂质，所选用的试剂不正确的是（  ）

某研究性学习小组用如图甲所示装置验证机械能守恒定律．让一个摆球由静止开始从A位置摆到B位置，若不考虑空气阻力，小球的机械能应该守恒，即 $\frac{1}{2}$ mv²=mgh．直接测量摆球到达B点的速度v比较困难．现让小球在B点处脱离悬线做平抛运动，利用平抛运动的特性来间接地测出v，如图甲中，悬点正下方P点处放有水平放置炽热的电热丝，当悬线摆至电热丝处时能轻易被烧断，小球由于惯性向前飞出做平抛运动．在地面上放上白纸，上面覆盖着复写纸，当小球落在复写纸上时，会在下面白纸上留下痕迹．用重锤线确定出A、B点的投影点N、M．重复实验10次（小球每一次都从同一点由静止释放），球的落点痕迹如图乙所示，图中米尺水平放置，零刻度线与M点对齐．用米尺量出AN的高度h₁、BM的高度h₂，算出A、B两点的竖直距离，再量出M、C之间的距离x，即可验证机械能守恒定律．已知重力加速度为g，小球的质量为m．

根据短文内容，完成下列句子，每空一词。

      Lu Xun is one of the greatest Chinese writers of the 20th century. Besides his famous stories, he also wrote many influential articles about the living conditions of Chinese people.

Born in 1881 to a wealthy family, Lu Xun had a happy childhood. In 1893, however, his grandfather, a senior government official, was put into prison for taking money. At the same time, his father became seriously ill. From that time on, his family were no longer accepted by their relatives and friends. These early experiences greatly influenced his writing.

By the time Lu Xun arrived in Nanjing to study at university in 1899, he already believed that Chinese society had to change and become modern. In 1902 he went to study in Japan. There, he began writing articles for several Chinese student magazines. He showed a gift for writing and translating and he even wrote several books, although none was popular. He returned to China in 1909 because he was in need of money.