根据短文理解, 从A、B、C三个选项中选择最佳答案填空。
Miss Richards was a teacher at a school for boys and girls. She 1 chemistry and physics from the lowest to the highest classes in the 2. Sometimes the new classes 3 rapidly, but sometimes they were very 4, and then Miss Richards had to 5things many times.
One year, the first class had been studying chemistry for several 6 when Miss Richards suddenly asked, "What is water? Who knows? 7 up!"
There was silence for a few seconds, and Miss Richards felt sad, but then one boy 8 his hand.
"Yes, Dick?" said Miss Richards encouragingly. He was not one of the brightest children in the class, so she was 9 that he could answer.
"Water is a liquid which has no 10 until you wash your hands in it, Miss. Then it turns black, "the boy replied with great confidence.
