鸡和兔子一共有13只，其中有4只兔子，那么有____只鸡。

我国北方自然带，从东到西呈现出森林→草原→荒漠的更替（　　）

根据下面的化学结构式回答问题．

（1）该式中，A表示____ ，B表示____ ，C表示____ ．

（2）该式表示____ 化合物，含有____ 　个肽键．

（3）该化合物由____ 个氨基酸分子失去____ 分子水形成，这种反应叫____ 

（4）该式中的氨基酸种类不同，是由____ 决定的．

（5）若氨基酸的平均相对分子量为90，则蛋白质的相对分子质量约为____ ．

   Mr. Smith is an English teacher. He comes from America. He has a daughter. Her name is Amy, and she is a good engineer in New York. Mr. Smith likes traveling very much, and he can speak Spanish very well. Now, he is in China, so he wants to learn some Chinese. He works very hard at his Chinese, and goes to Chinese classes every evening. He has a lot of friends in China and they often help him. He likes to talk with them. He tells his daughter that he can speak good Chinese, and he doesn’t have any trouble（麻烦） with his Chinese, but the Chinese have a lot.