请以《因为有了期盼》为题写一篇文章。

要求：①内容具体，有真情实感；②除诗歌外，文体不限；③文中不得出现真实的人名、班名、校名；④不得抄袭；⑤字数不少于600字。

     Mr. Jackson is a teacher of science in a middle school. His students like his classes because his classes are very interesting. They can learn a lot in his class. The students can make（制作）many things. His little son Val also likes making things. He makes lots of machines（机器）, such as a machine like a man. It can do the job that is dangerous（危险的）for people to do. He says, “I want to be a scientist.”
根据短文内容，选择正确答案。

美洲的印第安人属于

如图是一个古代车轮的碎片，小明为求其外圆半径，连接外圆上的两点A、B，并使AB与车轮内圆相切于点D，半径为OC⊥AB交外圆于点C．测得CD=10cm，AB=60cm，则这个车轮的外圆半径是 ____.

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