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正确答案C
