题目

Long ago, there lived in Egypt a learned man. He was so well-known   61   his knowledge of almost everything that lots of people from all over the country came to learn from him. The great man   62   (teach) his students whole-heartedly and answered their questions with great   63   (patient). One day, a student asked him, “My dear teacher, didn’t you say you yourself have many more questions about things than we do?   64   I think we students have far more than you.” With a smile on his face, the teacher drew two circles, one as large as a big cake, the other sm下列各组物质中，前者属于混合物，后者属于纯净物的是A.冰水混合物稀有气体B.清新的空气液态氧气C.氮气二氧化碳D.澄清的石灰水清澈的泉水