is this book ----- you borrowed from the library the other d

答案是D
is（this book）_____ you borrowed from the library the other day?
首先要明白主语是this book,这不是定语从句,因为没有先行词,所以A,B,C都不对,
只有what在句中能做双重成分,它既做is的表语,又做从句中borrow的宾语.
这是what引导的名词性从句
如果句子是is this（the book）_____ you borrowed from the library the other day?的话,that和which都能用