印度也是个人口众多的国家，其人口总数居世界（    ）

读“地震波速度与地球内部构造图”，回答问题。

敦煌莫高窟被称为世界最大的艺术宝库之一，主要是因为(     )

若变量$x,y$满足约束条件$\left\{\begin{array}{c}\begin{array}{c}x+y\le 8\\ 2y-x\le 4\\ x\ge 0\end{array}\\ y\ge 0\end{array}\right.$且$z=5y-x$的最大值为a,最小值为b,则a-b的值是(     )

     When you look up into the sky at night, have you ever felt that your eyes are playing tricks on (捉弄) you? It seems that the stars are moving all the time.
      Actually, there is nothing wrong with your eyes. This twinkling effect is called scintillation (交光). Scintillation happens because of air movements in the earth's atmosphere (大气). Light is “bent (弯曲的)” when it travels through different parts of the earth's atmosphere. As the air in the earth's atmosphere is moving all the time, the light from the stars looks as if it is moving too.
    The same thing also happens to things on the ground. On a very hot and shiny day, if you look at the road, the image in the distance is not clear and things move slightly. You can also see the same effect if you drop a rock into water. The rock appears a little unclear under the moving water.
      This twinkling effect causes a lot of problems for astronomers (天文学家) since they cannot observe the stars clearly. A telescope (望远镜) was sent into space so that the air movements in the atmosphere could be avoided (避免). It took a long time to build the space telescope but finally in 1990, a huge space telescope called the Hubble Space Telescope was successfully sent into space. Since then, astronomers have many important observations that have helped people understand space better.
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