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Then the Dean and the committee came back, “Gentlemen”, the Dean said, “the candidate we have chosen is Mr. Hobdell.” Uncle Theo had won. You could have knocked him down with a feather.  The audience were completely taken by surprise, and the Dean continued, “ I think I ought to tell you how we arrived at the decision. We were all filled with admiration at the learning and eloquence of Mr. Adams. I was greatly impressed. I didn't think he had it in him. But, you will remember, Mr. Adams read his lecture to us. When Mr. Hobdell’s turn came, he repeated that speech, word by word from memory, though, of course, he couldn’t have seen a line of it before. Now a fine memory is absolutely necessary for this post; and what a memory Mr. Hobdell must have! This is why we decided that Mr. Hobdell was exactly the man we wanted! ”

(1)  文中，“You could have knocked him down with a feather.” 为什么要用could have done的形式，可否改用一般过去时You knocked him down with a feather 呢？ 含义有何不同？

(2)  文中，为什么要说“ I didn't think he had it in him” 而不说“ I don't think he ...” ? 用I didn't think 和 I don't think 含义句意究竟有何不同？

(3)  文中，“he couldn’t have seen a line of it before” 可以换用一般过去时吗？ 这里用couldn't have done表达了讲话者什么含义呢？

(4)  文中，“ and what a memory Mr. Hobdell must have! ”    这里must是情态动词，说明讲话者非常肯定的推测吗？