The house, is blue, will be used as a library.A. whose the roof B. the roof of whoseC. the

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A. whose the roof
B. the roof of whose
(答案→)C. the roof of which
D. which roof
解析：“介词 + which / whom”引导的定语从句
关系代词在定语从句中作介词的宾语时，常常将介词前置于作宾语的关系词前，
I love the music that I can dance to. = I love the music to which I can dance.
The man who I talked to just now is my brother.= The man to whom I talked just now is my brother.
介词后面的关系词指人时只用whom, 不能用who或that; 指物时只用which,不能用that.
注意，并非所有情况下介词都可以前移，含有介词的短语动词不可拆开，介词仍需放在动词后面，如：look for, look after, take care of等。
“介词+ which / whom”前还可以用some, any, none, all, both, neither, many, most, each, few等代词，名词或数词等。
His sons, both of whom loved music very much, spent most of their money on CDs.
The house, the roof of which is blue, will be used as a library.
whose的用法：
指代人或物，在从句中作定语，表示“他/它的”：We all dislike the boy whose father is a manager.
这里也可以用whose roof；whose+名词=the+名词+ of which= of which the名词，故选C。