—Where did you get this book?
—I picked it up from a secondhand bookshop if you ________ know.
A.must B.will C.can D.shall
—Where did you get this book?
—I picked it up from a secondhand bookshop if you ________ know.
A.must B.will C.can D.shall
A [考查情态动词。if you must (do sth.)表示“如果你一定要(那么做)”,所以选A,表示“如果你一定要知道的话”。]