如图所示，A、B 两物块的质量分别为 2m 和 m，静止叠放在水平地面上．A,B 间的动摩擦因数为μ，B 与地面间的动摩擦因数为 $\frac{1}{2}$ μ．最大静摩擦力等于滑动摩擦力，重力加速度为 g．现对 A 施加一水平拉力 F，则（   ）

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   If you want to have a wonderful vacation, you can visit Kingston(金斯顿 ). It's a famous Canadian city on the bank of Lake Ontario (安大略湖). It is about three hours'  trip by bus from Kingston to Toronto(多伦多 ). There are some famous universities(大学)in Kingston. About 25,000 students study in these universities. Queen's University, founded in 1841, is more famous than any other university in Kingston. Kingston is one of the friendliest cities in the world and it welcomes tourists from all over the world. Wherever tourists go in this city, they will see a warm smile on the street or hear a happy "hello" in a public park. It's easy for them to feel at home

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