<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 about 15 minutes and that chain smoking is far more hazardous. Recent findings of medical research show that even a tiny drop of nicotine (about 3 grams in weight) can kill a large animal like a horse. Some medical reports have also proved that heavy smoking would seriously affect an adult’s sexuality, and smoking after a meal also disturbs one’s digestive system. Cigarette smoking not only jeopardizes the physical health of a smoker, it also affects the health of a nonsmoker who passively inhales the polluted air into his or her lungs when he or she is exposed to a winding shroud of cigarette smoke. The impact on such passive smokers (also called second-hand smokers), according to the experts in the medical field, can be even more damaging, and that is why smoking is usually prohibited in most public places. (211 words)