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     No one knows exactly how the earth began. However, according to a widely accepted theory, it

began   1  a "Big Bang". Then the earth exploded loudly and produced water vapor and many gases, 

   2  (make) the earth's atmosphere. And then as the earth cooled down, water appeared and stayed 

   3  the surface. It was the presence of water    4   allowed the earth to dissolve harmful gases and acids

into the oceans and seas,   5  made it possible for life to develop. And the   6    (appear) of small plants

encouraged the development of early shellfish and all sorts of fish. Next, green plants began to grow on

land.    7   were followed by land animals. Later, reptiles appeared for the first time, after which dinosaurs developed. However, they suddenly disappeared. Therefore, mammals   8   (rise) on the earth. Finally,

humans appeared and spread all over the earth. However, they are putting too much carbon dioxide into

the atmosphere, making the earth too hot to live on. So   9   life will continue on the earth will depend on

whether this problem can    10