_____you understand this rule ，you‘ll have no difficulty in dealing with such problems.
A、WhileD
once意思是“一旦”。while“当⋯⋯时候”;though“虽然”;unless“除非”。“一旦你明白了这个规则，处理类似的问题就不困难了。”显然，d符合句子意思的表达需要。
Theagriculturalrevolutioninthe19thcenturyinvolvedtwothings：theinventionoflabor-savingmachineryandthedeyelopmentofscientificagriculture.Labor-savingmachinerynaturallyappearedfirstwherelaborwasscarce.“InEurope，”saidThomasJefferson，“theobjectistomakethemostoftheirland，laborbeingabundant：hereitistomakethemostofourlabor，landbeingabundant.”ItwasinAmerica，therefore，thatthegreatadvancesin19thcenturyagriculturalmachineryfirstcame.
Attheopeningofthecentury，withtheexceptionofacrudeplowfarmerscouldhavecarriedpracicallyalloftheexistingagriculturalimplementsontheirbacks;by1860，mostofthemachineryinusetodayhadbeendesignedinanearlyform.Themostimportantoftheearlyinventionswastheironplow.Asearlyas1790CharlesNewboldofNewJerseyhadbeenworkingontheideaofacast-ironplowandspenthisentirefortuneinintrodueinghisinvention.Thefarmers，however，wouldhavenoneofit，claimingthattheironpoisonedthesoilandmadetheweedsgrow.Nevertheless，manypeopledevotedtheirattentiontotheplow，untilin1869JamesOliverofSouthBend，Indiana，turnedoutthefirstchilled-stillplow.?
1.Whatisthemaintopicofthepassage?
a.Theneedforagriculturaladvancestohelpfeedagrowingpopulation.
b.Thedevelopmentofsafermachinesdemandedbythelabormovement.
c.Machinerythatcontributedtotheagriculturalrevolution.
d.NewJerseyasaleaderintheagriculturalrevolution.
2.Theexpression“makethemostof”inLine4isclosestinmeaningtowhichofthefollowing?
a.getthebestyieldfrom
b.raisethepriceof
c.makefulluseof
d.earnalivingon
3.WhichofthefollowingcanbeinferredfromwhatThomasJeffersonsaid?
a.EuropewaschangingmorequicklythanAmerica.
b.EuropehadgreaterneedoffarmmachinerythanAmericadid.
c.Americawasfinallyrnningoutofgoodfarmland.
d.TherewasashortageofworkersonAmericanfarm.
4.Itcanbeinferredthatthewaord“here”inLine4refersto_____.a.Europe
b.America
c.NewJersey
d.Indiana
5.Whatpointistheautormakingbystatingthatfarmerscouldcarrynearlyalltheirtoolsontheirbacks?
a.Farmershadfewtoolsbeforetheagriculturalrevolution
b.Americansweretraditionallyself-reliant.
c.Lifeonthefarmwasextremelydifficult.
d.Newtoolsweredesignedtobeportable.
I ‘ve been telling you_____not to do that ，but you neve listened to me.That ’s why you gotinto trouble.
Copernicus maintained that the earth_____round the sun.
It was not until she had finished all her work_____.
If he_____the storekeeper ‘s scissors ，he would have forgotten to buy a pair.
