阅读并选择。
Bob is a lovely schoolboy. He likes playing football very much. He often plays football with his classmates after school, but he never goes home too late. He usually gets home between 4:30 and 5:30 in the afternoon. One day he comes back home late, so his mother says, ”You come back home late today, Bob!””Yes, we have got a new teacher. He teaches us Maths,” Bob answers. “How is he?”his mother asks. “I don't know. I think I can't believe him.””Why?”his mother says. “In today's Maths class, one moment he says two and three is five. But the next moment he says one and four is five.” Bob answers.
上清液的放射性很高；沉淀物的放射性很低。