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Manypeoplebelievetheglarefromsnowcausessnow-blindness.Yet,darkglassesornot,theyfindthemselvessufferingfromheadachesandwateringeyes,andevensnow-blindness,whenexposedtoseveralhoursof"snowlight".
TheUnitedStatesArmyhasnowdeterminedthatglarefromsnowdoesnotcausesnow-blindnessintroopsinasnow-coveredcountry.Rather,aman'seyesfrequentlyfindnothingtofocusoninabroadexpanseofbarrensnow-coveredterrain.Sohisgazecontinuallyshiftsandjumpsbackandforthovertheentirelandscapeinsearchofsomethingtolookat.Findingnothing,hourafterhour,theeyesneverstopsearchingandtheeyeballsbecomesoreandtheeyemusclesache.Natureoffsetsthisirritationbyproducingmoreandfluidwhichcoverstheeyeball.Thefluid
coverstheeyeballinincreasingquantityuntilvisionblurs,thenisobserved,andtheresultistotal,eventhoughtemporary,snow-blindness.
ExperimentsledtheArmytoasimplemethodofovercomingthisproblem.Scoutsaheadofamainbodyoftroopsaretrainedtoshakesnowfromevergreenbushes,creatingadottedlineastheycrosscompletelysnow-coveredlandscape.Eventhescoutsthemselvesthrowlightweight,darkcoloredobjectsaheadonwhichtheytoocanfocus.Themenfollowingcanthenseesomething.Theirgazeisarrested.Theireyesfocusonabushandhavingfoundsomethingtosee,stopscouringthesnow-blanketedlandscape.Byfocusingtheirattentionononeobjectatatime,themencancrossthesnowwithoutbecominghopelesslysnow-blindorlost.Inthiswaytheproblemofcrossingasolidwhiteterrainisovercome.
Topreventheadaches,wateringeyesandblindnesscausedbytheglarefromsnow,darkglassesare__________. A．indispensible
B．useful
C．ineffective
D．available
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