根据短文内容,选择正确答案。 Sam is a teacher of maths. He is not young, but he is not old. He is 40 years old. He has a round (圆的) face and black hair. He is short. There are forty students in his class. They all like him. Now it's in the afternoon. Look, some students are studying in the classroom. Sam is there, too. He is helping them to study maths. He is a good teacher, and he is a good friend of his students.