3、_____________thereonlyonce.三、单项选择。1、Both hisparentslooksad.Maybe they ______what'shappenedtohim.A. knew B. haveknown C. mustknow D.willknow2、He has _____been to Shanghai,has he?A.already B.neverC.everD.still3、Have you met Mr Li_____? A. just B. ago C.before D.am