(·成都第二次诊断)—Where did you get this book?—I picked it up from a second­hand bookshop

问题补充：

(·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. must

B. will

C. can

D. shall

解析：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”。