题目内容：
根据下面材料，回答题。Nowadays，air travel is very_____21_______ ．We are not surprised when we watch on TV that a politician has talked with French President in Paris and attended ameeting in Beijing on the same day．_____22_____，if a person takes long．distance flying frequently，he can be so tkedthat he maybe feel his brain is in one country，his _____23_______ in another．______24______，he ______25_____ knows where he is．
When weny from east to west or_____26_____ versa，the_____27_______we experience after taking a longdistance flying is______28______，because we cross timezones．According to doctors，airtravelers，after crossing several time zones，are in no ______29____to go to work，and they should go _____30_______to bed ______31______arfival．
As toairline pilots who often live _____32_____ their own watches and ignore local time，there is no need for themto worry about their health although they sometimes have breakfast at midnight，because they are used to_____33_____and are _____34_____fit．
Manybusinessmen like longdistance flights，thinking they are______35______ to have been chosen and they areout for promotion．They are lucky if the company follows the doctor's advice and allowthem to rest for a day or two______36______working．
However，sometimes the manager isso energet ic that he befieves everyone is _____37_______ to be as fit as he is．Since he has never felt_____38_____after flying himself，the work he assigns is so______39______ that the employee is too_____40_____to carry the work out satisfactorily．That is disastrous for theemployee's health and the company's reputation．
第21答案为__________ A．ordinary
B．common
C．plain
D．usual
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