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    Skin­diving is a new sport today. This sport takes you into a wonderful new world. It is like a visit to the moon. When you are under the water, it is easy for you to climb big rocks, because you are no longer heavy.

    Here, under the water, everything is blue and green. During the day, there is plenty of light. When fish swim nearby, you can catch them with your hands.

When you have tanks of oxygen on your back, you can stay in deep water for a long time. But you must be careful when you dive in deep water.

    To catch fish is one of the most interesting parts of this sport.

    What's more, there are more uses for skin­diving. You can clean ships without taking them out of water. You can get many things from the deep sea.

    Now you see that skin­diving is both useful and interesting.

国际研究小组借助于智利的甚大望远镜，观测到了一组双星系统，它们绕两者连线上的某点O做匀速圆周运动，如图所示．此双星系统中体积较小成员能“吸食”另一颗体积较大星体表面物质，达到质量转移的目的，被吸食星体的质量远大于吸食星体的质量．假设在演变的过程中两者球心之间的距离保持不变，则在最初演变的过程中（   ）