D
i don't believe you have already finished reading the book--i _________ it to you this morning!
应该填的空前的那一句话是线索,i don't believe you have already finished reading the book.我不相信你已经把这本书看完了.
i _________ it to you this morning!
感叹号表示惊叹,说明不相信的原因.
A would lend B was lending C had lent
这三个选项都不对,A表示将要借书给你,那前面就不可能说书读完了.B更不可能,现在正借书给你.C是表示过去的过去才用.
所以正确答案是D,表示今天早上借给你,你就看完了.过去式.