问题
选择题
(2013·成都第二次诊断)—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
答案
答案：A
选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”。