【①Whojumpshighestinyourclass?—john____.A.hasdoneB.didC.doesD.do我认为是选B可答案是选C、可跳高不已经是过去式了吗?②Todaytheforestshavealmostgone.Peoplemust____downtoomanytress.A.stoponcutB.stopfromc】

　　①Whojumpshighestinyourclass?

　　—john____.

　　A.hasdoneB.didC.doesD.do

　　我认为是选B可答案是选C、可跳高不已经是过去式了吗?

　　②Todaytheforestshavealmostgone.Peoplemust____downtoomanytress.

　　A.stoponcutB.stopfromcuttingC.bestoptocutD.bestopfromcuttingC

　　③Wemustdoeverythingwecan____wastewaterfromrunninginto

　　rivers.

　　A.keepB.keptC.tokeepD.keeping

　　can不是情态动词吗?后面难道不是接原型吗?

　　④Shecounldn't__herwords___bythestudents.

　　A.getunderstandB.getunderstoodC.maketounderstandD.makeunderstood

　　⑤—Shallwegeforawalkthisevening?

　　—Soryy.I'mafraidnot.Iamtoobusy___out.I'mbusy__mybike.

　　A.togoB.goingmendingC.goingtomendD.togomending

　　⑥—WhatshouldwedotofightA/H1N1flu?

　　—Weshouldwashhandsoften,avoid__tocrowdedplacesandsoon.

　　A.goB.goingC.goneD.togo

　　还有就是非谓语动词的那部分知识我不太明白

　　希望可以帮我讲一下.