请教nn一道语法题...谢谢:)

Cooperative apartment houses have the peculiar distinction of being dwellings that must also operate as businesses
    

A of being dwelling that must also operate as businesses

B  of dwellings that must also operate like business

C that they are dwellings that must operate like business

D that , as dwellings, they must also operate like businesses

E to be a dwelling that must also operate as a business

为什么是答案a 呢? e怎么不行

参考此链接

http://forum.chasedream.com/dispbbs.asp?BoardID=23&replyID=357043&id=43384&skin=0

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