17.Thisdiseaseissecondonly_______heartattackasacauseofdeathallovertheworld.A.toB.ofC.withD.from18.Thewordisnewtous.You'dbetter______inthedictionary.A.lookatitB.lookforitC.lookitupD.lookitout19.Ifthereisnoon

　　17.Thisdiseaseissecondonly_______heartattackasacauseofdeathalloverthe

　　world.

　　A.toB.ofC.withD.from

　　18.Thewordisnewtous.You'dbetter______inthedictionary.

　　A.lookatitB.lookforitC.lookitupD.lookitout

　　19.Ifthereisnoone_______himwiththepreparation,hewillhavetoputoffthe

　　meeting.

　　A.helpB.tohelpC.helpsD.helped

　　20.Assoonasshe_______opentheletter,Mrs.Whitewillfindoutwhathashappenedto

　　herhusband.

　　A.tearsB.istearingC.willtearD.hastorn

　　21.______thegovernmentargeestogiveextramoney,thetheatrewillhavetobeclosed

　　nextmonth.

　　A.UnlessB.IfC.SinceDAs

　　答案是17.A18.C

　　19.B20.A21.,可我不理解,麻烦帮我解答下