题目内容：
根据以下内容回答题。 Nowadays,airtravelisvery21WearenotsurprisedwhenwewatchonTVthatapoli-ticianhastalkedwithFrenchPresidentinParisandattendedameetinginBeijingonthesameday．
22,ifapersontakeslong-distanceflyingfrequently,hecanbesotiredthathemaybefeelhisbrainisinonecountry,his23inanother．24,he25knowswhereheis．
Whenweflyfromeasttowestor26versa,the27weexperienceaftertakingalong-distanceflyingis28,becausewecrosstimezones．Accordingtodoctors,airtravelers,aftercrossingseveraltimezones,areinno29togotowork,andtheyshouldgo30tobed31arrival．
Astoairlinepilotswhooftenlive32theirownwatchesandignorelocaltime,thereisnoneedforthemtoworryabouttheirhealthalthoughtheysometimeshavebreakfastatmidnight,be-causetheyareusedto33andare34fit．
Manybusinessmenlikelong-distanceflights,thinkingtheyare35tohavebeenchosenandtheyareoutforpromotion．Theyareluckyifthecompanyfollowsthedoctor'sadviceandal-
lowthemtorestforadayortwo36working．However,sometimesthemanagerissoenerget-icthathebelieveseveryoneis37tobeasfitasheis．Sincehehasneverfelt38afterfly-inghimself,theworkheassignsisso39thattheemployeeistoo40tocarrytheworkoutsatisfactorily．Thatisdisastrousfortheemployee'shealthandthecompany'sreputation．
第21题应选 A．ordinary
B．common
C．plain
D．usual
参考答案：
答案解析：