鸦片战争后，中国的茶叶和生丝的出口量猛增。这一现象说明的实质性问题是

假设你刚来到新的班级，要向大家做一个个性化的自我介绍，你会怎么说？（400字左右）

实验室常用下列装置来制取、收集气体．请回答以下问题．

如图14所示，质量为m的物体A静止在地面上，上表面带有弯钩。一把弹簧秤连接有挂钩的拉杆和弹簧相连，并固定在外壳一端O上，外壳上固定一个提环。可以认为弹簧秤的总质量集中在外壳上为m₀，弹簧和拉杆质量忽略不计，弹簧的劲度系数为k。手拿弹簧秤的提环，秤钩钩住物体A，秤体保持竖直，开始弹簧没有形变，示数为零。现竖直向上缓慢提起，当提环提升距离L₁时，A刚要离开地面，此过程手做功W₁、手做功的平均功率为P₁；若加速向上提起，提环上升的距离为L₂时，A刚要离开地面，此过程手做功W₂、手做功的平均功率为P₂。假设弹簧秤示数一直在量程范围内，则（   ）

阅读下列短文，从文后各题所给的四个选项中选出一个最佳答案。

    1 years and years people have been saying that railways are dead. “We can do 2  railways,” people say, 3 cars and planes have made railways unnecessary. We all keep hearing that trains are slow, 4  money and are dying. But this is far from the 5 . In these days of expensive 6, railways have become highly competitive（竞争的）with cars and planes. If you want to carry people or things from place to place, they're 7 than planes. And they have much in 8 with planes. A plane goes in a straight line and 9 does a railway. What is more, a railway takes you from the heart of a city center. It doesn't stop as a car does on crowded roads. And no plane or car can ever carry as many goods(货物) as one train 10.

    Far from being dead, railways are much 11. Modern railway lines give you a comfortable journey. Where else can you eat well, sleep comfortably, feel safe and enjoy the scene while you are traveling 12? And we are only the beginning, for we have just  13 the age of super-fast trains, which can travel almost 200 14 an hour or more. Soon we will be wondering 15 we spent so much on highways and planes we can't fly in because we don't have enough money to buy the oil.