There are three classes of structure: primary, secondary and tertiary.
答案解析
解析：
为了更好地理解这个知识点，我们可以通过生动有趣的例子来帮助记忆。比如，我们可以把这三类结构比喻成搭建房屋的过程。主要的结构就像是房子的主体结构，比如墙壁和屋顶，没有这些主要结构，房子就无法存在。次要的结构就像是房子的装饰和附属设施，比如窗户和门，虽然不是主体结构，但是也很重要。而第三位的结构则是房子的一些额外设施，比如花园和车库，虽然不是必需的，但是可以提升房子的价值和舒适度。
相关知识点：
结构分主要次要第三位
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