任何一篇文章都围绕某个中心展开，这个体现中心的词叫关键词。经常作者要不断重复关键词使文章连贯，例如：
Early this term we did a survey among the senior middle school students concerning their usage of mobile phones. The survey shows that eighty-seven percent of them have a mobile phone. The survey also shows that the majority of the owners use mobile phones to make phone calls. However, while more girls use their mobile phones to send text messages,
