minutes and that chain smoking is even more hazardous.
4． Smoking after a meal can also disturb one’s digestive system.
5． Heavy smoking can seriously affect an adult’s sexuality.
6． Smoking does harm to second-hand smokers as well.
7． Smoking makes one’s mouth smell.
8． The life expectancy of smokers is shorter than that of nonsmokers.
9． Smoking in bed may cause fire..
根据上面所列出的七个句子，我们可以选择以下两个主题句，即：
<1> Smoking is a bad habit.
<2> Smoking is a bad habit that does a lot of harm to human health.
第一个主题句表明了作者的态度，但不如第二个具体，因此在展开中心段落时不仅要谈吸烟对人体健康的不良影响，还要涉及吸烟在其它方面造成的危害。这样的段落考生较难掌握。因此主题句越是相对地具体段落就越好展开。但这并不等于说第一个主题句不好；如果主题句太过具体，段落可展开的范围就会过窄。如果范围较窄，有些考生所能列出的例证就会相对较少，整篇文章写下来后可能达不到所要求的字数。因此选择哪一个主题句要视具体情况而定。
用于支持段落主题句的论据其实就是我们开始列出的那些例证，但应视主题句所涉及范围的大小而有所取舍。下面我们就上面的两个主题句分别构建两个不同的中心段落：
(1)
Smoking is a bad habit. For one thing, smoking in bed may cause a terrible fire. For another, regular smoking makes one’s mouth smell bad. And worst of all, cigarette smoking does great harm to human health. In many cases, cigarette has proved to be the direct cause of such a fatal disease as cancer-hence its nickname, "Cancer Stick". Experts say that smoking one cigarette can reduce one’s life by