Children today have many things to play with. They can watch television, surf ____ Internet, or play video games. But what do you think children ____ (do) for fun long ago? They read books.

    Some of you might ask, "Are books ____ (real) fun?" Believe it or not, books provide many things that television or computers cannot. Since books use only words ____ (tell) a story, they can improve your imagination. You can picture the characters and background in your head and you can also imagine ____ the characters feel.

    The words and sentences that are used in storytelling are also very interesting and usually cannot be found often on television or on websites. They are sometimes so beautiful and touching ____ they can bring tears to your eyes.

    Books also help us to think harder and ____ (deep). As we continue ____(read), we ask questions to ____(our) and try to find answers.

    These are some of the ____(reason) why books are so fun to read.

    Remember: the more you read, the more you grow.

已知，气体D的大量生产曾经解决了地球上因粮食不足而导致的饥饿和死亡问题，气体D和F反应可生成盐．气体D和A溶液反应生成白色沉淀．气体C为无色气体，气体E为红棕色气体．B溶液加入过量的F溶液能生成A溶液．

请回答下列问题：

某一行星有一质量为m的卫星，已知卫星的轨道半径为R，周期为T，万有引力常量为G．求：

阅读理解。

    For years and years people have been saying that railways are dead. “We can do without railways,” people say—as if cars and planes have made railways unnecessary. We all keep hearing that trains are slow, lose money and are dying. But this is far from the truth. In these days of expensive oil, railways have become highly competitive with cars and planes. If you want to carry people or goods from place to place, they are cheaper than planes. And they have much in common with planes. A plane goes in a straight line and so does a railway. What is more, a railway takes you from the heart of a city center. It doesn't stop you as a car does on crowded roads. And no plane or car can ever carry as many goods as one train does.

    Far from being dead, railways are much alive. 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 at the same time? And we are only the beginning, for we have just entered the age of super-fast trains, which are traveling 150 miles an hour and more. Soon we will be wondering why we spent so much on highways and planes we can't fly in because we don't have enough money to buy the oil.