用方框中所给单词的适当形式填空。

         As we know, there are many beautiful places in China, such as Hong Kong, Harbin. They attract a lot of people at home and abroad. But it's hard for ____ to choose where they can go. You can tell them ____ Guide to China, which ____ China in detail. If they have no chance to go through the book, you may show them advertisements for traveling, then after ____, they can decide where to go. They will have a good trip and take photos in ____ of it.

①在点燃H₂，CO，CH₄等易燃气体前，必须检验气体的纯度

②在稀释浓硫酸时，应将浓硫酸沿器壁慢慢注入水中并不断用玻璃棒搅拌

③浓碱液对皮肤有腐蚀性，如果不慎沾到皮肤上，应用较多的水冲洗，再涂上硼酸溶液

④给试管中的液体加热时，液体体积应不超过试管容积的三分之一

⑤点燃添满酒精的酒精灯

⑥给试管中的液体加热时，试管口不能对着自己或他人．

如图所示，一个长直轻杆两端分别固定一个小球A和B，两球质量均为m，两球半径忽略不计，杆的长度为。先将杆AB竖直靠放在竖直墙上，轻轻振动小球B，使小球B在水平面上由静止开始向右滑动，当小球A沿墙下滑距离为$\frac{l}{2}$时，下列说法正确的是(      )