请认真阅读下面短文，并根据短文内容回答问题。
Dear Helen,
      Thanks for your last letter. Yes, I'm enjoying myself on my exchange program in France. It's even better than I thought it would be. I was a bit nervous before I arrived here, but there was no reason to be. My host family are really nice. They go out of their way to make me feel at home. And you wouldn't believe how quickly my French has improved! I'm very comfortable speaking French now. Although I still make lots of mistakes, it doesn't make me worried as it used to. My biggest challenge (挑战) is learning how to behave (表现) at the dinner table. As you can imagine, things are really not the same as the way they are at home. For example, you shouldn't put your bread on your plate. You should put it on the table! I thought that was very strange at first, but now I'm used to it. You shouldn't eat anything with your hands except bread, not even fruit! (You have to cut it up and eat it with a fork.) Another thing is that it is very rude to say you're full. If you don't want any more food, you should just say, “It was delicious.” Also, it's rude to put your hands in your lap. You should always keep your hands, but not your elbows (肘部), on the table. I have to say, I find it difficult to remember everything, but I'm gradually getting used to things and don't find them so strange any more.
      I'll write soon and tell you more about life in France. Hope you're having a good school year.
Yours,
Li Ying

如图所示，质量为m的物体，在水平力F的作用下，沿倾角为α的粗糙斜面向上做匀速运动，物体与斜面间的动摩擦因数为μ．则水平推力的大小为（　　）

甲午战争期间，民众甚至为了几个劳逸费，帮助日军搬运炮弹。1938年春，台儿庄战役打响。面对日军的凶猛进攻，负责指挥作战的李宗仁和白崇禧邀请周恩来到其寓所共话战局。李宗仁指挥战前一向都拥兵自重的“杂牌军”在枪林弹雨中浴血奋战，他们放下成见、放下自私，携手共赴国难；中共则组织鲁南老百姓，冒着炮火，把大刀、弹药、粮食、蔬菜、肉蛋、烧酒、食盐等送上前线，再将大批伤病员运往后方救治。……六里石村，百多户人家，只不过是京杭大运河畔的小村落，却是大战时的后勤基地之一，一桶桶开水，一罐罐小米稀饭从这里送往前线。

——摘编自《南方周末》2015年4月9日第1624期《抗战地理二》

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