3〕含有讨论说明或分析的问题通常既要有主要推展素材，又要有次要推展素材。
        2．4 写好推展句的方法
         　　主题句及关键词确定后，开始选择和主题有关的信息和素材。实质上，针对关键词测试每一个所选择的素材就是一个分类的过程。有一种常用的方法就是句子展开前加以设问，然后解答，即设问－解答(why-because)的方法。下面我们通过举例来看一看这种分类过程是如何完成的。例3：
        　　假设 (suppose) Topic sentence: English is an international language?
        　　设问(why) Why can we say English is an international language?
        　　解答(because)
        　　Because: English is spoken by pilots and airport control operators on all the airways of the world.
        　　Because: Over 70 percent of the world's mail is written in English.
        　　Because: More than 60 percent of the world's radio programs are in English.
        　　Because: Pakistan was once one of the British colonies.
        　　Because: Many members of the International Language Society are selected from English-spoken countries.
         　　从上面可以看出，最初的三个推展句是和关键词an international language一致的。第四句和主题句不统一，尽管第四句会成为另一主题句(Many Pakistanis speak English)很好的推展句。第五句说服力不强。
        　　当然，在写成段落时，没有必要在每个句子开头写上 "because"，但是，在动笔展开句子时，头脑里要想着这个词，这种检验方法能帮助你把注意力集中在某个重心，围绕主题思想层层展开。