Now we're lucky because prepositions are a closed group in English, a closed group.
现在我们很幸运，因为介词在英语中是一个封闭组，一个封闭组。
Closed group
原声例句
可汗学院公开课：英语语法
英语百科
Algebraically closed group
In mathematics, in the realm of group theory, a group 
is algebraically closed if any finite set of equations and inequations that "make sense" in already have a solution in . This idea will be made precise later in the article.
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