而中心句的写法也有讲究。中心句必须能高度概括所在段落的论据，它的关键词应该在每—个论据中都有重复或适当体现。那种无关痛痒的叙述或说明性的句子，是不适宜用作中心句的。例如：
1.Students always feel relaxed and happy during breaks.(叙述性句子)
2.Break times are scheduled for about 10 minutes.(说明性句子)
以下即是论述“学校规定‘课间学生只能呆在自己的班级里’对吗?”的一篇学生习作：
Although some people believe that students should stay in their own classrooms during break times，I would like to argue that we should be allowed to spend break times in another class.
The most important reason for believing that is that many students have friends in other classes.We spend all day in our own classroom，and break times are the only time we have to spend with other friends.It can become very tedious(令人厌倦的)to have to spend even more time with the same people.
A further reason for allowing student to choose where they spend their break times is that it would stop arguements.If students are forced to spend time with classmates who are not good friends，they can annoy each other.This leads to problems that have to be sorted out by teachers.
Teachers argue that we all should stay in our own classes，because it is then easier to know what is going on.They say that it is difficult to keep track of students when they are walking round the corridors.However，students could be given the chance to choose a different classroom to spend the whole break time in.That would mean that there would not be any students in the corridors.
As I have explained，although it might be a little easier to manage when everyone stays in their own classroom，it would make break times happier for all students if they were allowed to choose where they spent their time.
这篇范文符合“四块论”的基本模式，正反论述兼顾，结构严谨，中心突出。
