阅读下面一首诗，完成题目。
                                                                                                                       宿云梦馆（欧阳修）
                                                                                                               北雁来时岁已昏，私书归梦杳难分。
                                                                                                               井桐叶落池荷尽，一夜西窗雨不闻。

古诗文阅读。
                                                                                                   春望
                                                                                                     杜甫
                                                                                     国破山河在，城春草木深。
                                                                                     感时花溅泪，恨别鸟惊心。
                                                                                     烽火连三月，家书抵万金。
                                                                                     白头搔更短，浑欲不胜簪。

如图甲所示，当用指甲剪剪指甲时，指甲剪可以看成是多个杠杆的组合．乙中画出剪指甲时其中一个杠杆DBE的动力臂L₁和阻力F₂

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