制作泡菜时要用特殊的坛子，坛子口必须加水密封，其目的是（　　）

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Dear Jenny,

   How are you? I am visiting my cousin Howard in Australia. It is hot in Hong Kong but it is cold in Australia in June. When it is summer in Hong Kong, it is winter in Australia. It is snowing heavily now. I like snow a lot and it is a white world outside. I will play with my cousin in the snow this afternoon. We want to make a snowman. I think it is very interesting!

   I like my cousin's house. It is big and there is a beautiful garden（花园）. We can play ball games. My cousin's neighbors are friendly but they cannot speak Chinese. I talk with them in English.

   See you next month.

 Tony

如图所示，固定斜面的倾角θ=30°，物体A与斜面之间的动摩擦因数为μ，轻弹簧下端固定在斜面底端，弹簧处于原长时上端位于C点，用一根不可伸长的轻绳通过轻质光滑的定滑轮连接物体A和B，滑轮右侧绳子与斜面平行，A的质量为2m，B的质量为m，初始时物体A到C点的距离为L．现给A、B一初速度v₀使A开始沿斜面向下运动，B向上运动，物体A将弹簧压缩到最短后又恰好能回到C点．已知重力加速度为g，不计空气阻力，求此过程中：

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