题目内容：
根据以下材料，回答题Manypeoplewhoflyatleastoccasionallyhavecomedownwithacoldortheflushortlyafterdisembarking.Istheairinairbornecommercialjets21?TheAssociationofFlightAttendants(AFA),22
which42,000flightattendantswith27airlinesarerepresented,evidentlythinksso.Theorganizationclaimsthattheincidenceofair-quality-relateddiseaseshas23amongitsmembersanddemandsthatpromptactionsbe24toimprovetheconditionsintheairplanecabin.AstudytheAFA25in1997uncoveredabout1,000self-reportedincidentsofheadaches,dizzinessandmemoryloss26flightattendantsandpassengers.someflightattendantsweretooillto27theirsafetyduties,whileothershavebeenpermanentlydisabled.
Becauseofairlines'effortsto28theirexpenses,cabin-airfiltersarenotcleaned29
Thecomplaintsofflightattendantsdonotalwaysgive30tocorrectmaintenance.Airlinesturnedtorecycledair,31thattheywouldreducesomeoftheircosts.Theyarenotrequiredtoputfiltersin.
Airlinesare32greatpressuretogettheirflightsout33sotheydonotpayasmuchattentiontosystemsthatarenotas34toflightscheduleandsafety.
Recentresearchfindingsemphasizetheconcernthatfilterscan35enginechemicalsintothecabinair.Thismaynothappen36everyflight,butitisapersistentproblem.
InastudypublishedinOctober1998aninvestigationwasmade37complaintsofcrewmembers38airqualityandhealth.Morethanhalfofthe200subjectsreportedhealthproblemsthey39tocabin
air.Itwasconcludedthatthesehealthproblemswereconsistentwith40harmfulgasesandsubstances.
A．humid
B．unhealthy
C．unbearable
D．crisp
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