请教八年级（上）英语寒假作业中几道难题：
1.-Listen!Is Porfessor Johnson giving a report in the hall?
-No,it____be him.He has gone to Japan.
A.needn't B.may not C.mustn't D.can't
2.The teacher has already told me but I'm still not clear____to do next.
A.howB.whatC.why D.interest
3.Mind your steps,boy! Don't you see the words“____the grass"
A.Keep off B.Keep out C.Take offD.Take out
4.All of us find ____necessary to take exercise every day.
A.this B.thatC.itD.them
5.I don't like the trousers.Please show me another____.
A.pieceB.pairC.oneD.ones
6.I don't think it____.
A.matterB.does matters C.mattersD.is matter
7.____beautiful sunshine we have!
A.howB.How aC.WhatD.What a
8.Ann drew____horse on the blackboard.
A.very a nice B.quiet a niceC.a quite nice D.very nice a
9.Lucy can't help Tom to mend____bike____.
A.her;himselfB.him;herselfC.his;herself D.her;herself
10._I like strawerries very much.
-_____
A.So does Gina B.Does so GinaC.So Gina doesD.Gina does so
这些题目的答案我全都知道,请给出解析和考点