开放作文。

     请根据下面提示，写一篇短文。词数不少于50。

     In your spoken English class, your teacher shows you the following picture. You are asked to describe

the picture and explain how you understand it.

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我国学者把中国第四纪黄土地层划分为下更新统的（）、中更新统的（）和上更新统的（）。