Manypeoplebelievetheglarefromsnowcausessnow-blindness．Yet,darkglassesornot,theyfindthemselvessufferingfromheadachesandwateringeyes,andevensnow-blindness,whenexposedtoseveralhoursof"snowlight"． TheUnitedStatesArmyhasnowdeterminedthatglarefromsnowdoesnotcausesnow- blindnessintroopsinasnow-coveredcountry．Rather,aman'seyesfrequentlyfindnothingtofocusoninabroadexpanseofbarrensnow-coveredterrain．Sohisgazecontinuallyshiftsandjumpsbackandforthovertheentirelandscapeinsearchofsomethingtolookat．Findingnothing,hourafterhour,theeyesneverstopsearchingandtheeyeballsbecomesoreandtheeyemusclesache．Natureoffsetsthisirritationbyproducingmoreandfluidwhichcoverstheeyeball．Thefluidcoverstheeyeballinincreasingquantityuntilvisionblurs,thenisobserved,andtheresultis total,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
正确答案是C