PassageTwo
Questions26to30arebasedonthefollowingpassage.
Inourdayoftheautomobileandpavedhighwayfewpeopleeverencounterquicksand(流沙).Yetquicksandisstillcommoninmanypansofthecountr y.Itmaybemoredangerousforbeinglessfamiliar.
Quicksandisusuallyfoundalongtheshoresandinthebedsofrivers.Itissimplysandsaturated(使饱和)withwaterfrombeneath，asfromaspring.Thewaterflowingintothesandseparatesthegrains.Thesuspendedgrainsgiverathereasilyandaheavyobjectplacedonthesurfaceislikelytosink.Howfastitsinksdependsonitsweightandsurfacearea.
Howdoesonedetectquicksand?Itcannotbedonebytheeyealone，sincesandwhichlooksfirmmaysuddenlycollapseandtr apanyonewhoventuresoutonit.Theonlywaytobesureistotestthesandbeforewalkingonit.Fortestaprobing(探索)poleorlongstickshouldbeused.Ifthepolesinksmorethansixinches，thesandisprobablyquicksand.
Atr avelerwhostumblesintoquicksandwillsoonsinktothedepthofhisknees.Ifhestandsstillorstr uggleswildly，hewillsinkevenfurther.Heshouldatoncelieonhisbackandstr etchouthisarms.Contr arytopopularnotion，quicksanddoesnotsuckobjectsdown，andwillsupportmoreweightthanwateralone.Whilethetr appedperson“floats”onthesurfaceofthesand，rescuersshouldbuildaplatform.withboardsorbranches.Thentheycanpullhimoutslowly.
Ifthetr appedpersonisalone，hecanrescuehimself.Whenheisinthefloatingposition，heshouldbeginrollingtowardsolidground.Rollingistheonlywayofgettingfree.Itshouldbedonewithfrequentr ests，sothatthetr appedpersondoesnottirehimself.Whenhereachessolidground，heshouldswinghislegstosafety，andquicklyscrambleoutofthequicksand.
Themainideaofthispassageis______.
A.thattodayfewpeopleeverencounterquicksand
B.thatquicksandisstillcommoninmanypartsofthecountr y
C.thatquicksandcannotbedetectedbytheeyealone
D.expressedbynoneoftheabove
正确答案：D[解析]A，B，C三选项点到但均未深入，故选D。
