After a very long time,as the amount of oxygen in the atmosphere increased,the earth became a more suitable place for plants to develop. Plants produced more oxygen which was freed into the atmosphere. Later on animals developed.

 
  • 经过很长的时间,由于大气层中氧含量的增加,地球变成更适合于植物生长的地方,植物生成更多的氧,逸入大气层,后来,出现了动物。
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