可以用topicsubject元素的hre f属性来表示主要主题，这个元素还包含subjectref元素来表示次要主题。

You can identify a primary subject with the href attribute of the topicsubject element, which also contains subjectref elements for the secondary subjects.

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希腊经常发生自然灾害，这些灾害主要是由于地震引起的，其他一些更为次要原因还包括洪水、森林火灾、暴风雨等(台风不是问题)。

Greece often faces natural catastrophes, mainly due to earthquakes and to a much lesser degree floods, forest fires, storms e. t. c. (Typhoons are not a problem).

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