问题详情

Many people who fly at least occasionally have come down with a cold or the flu shortly after disembarking. Is the air in airborne commercial jets【21】______？ The Association of Flight Attendants (AFA), 【22】______which 42, 000 flight attendants with 27 airlines are represented, evidently thinks so. The organization claims that the incidence of air-quality-related diseases has【23】______among its members and demands that prompt actions be【24】______to improve the conditions in the airplane cabin.
A study the AFA【25】______in 1997 uncovered about 1, 000 self-reported incidents of headaches, dizziness and memory loss【26】______flight attendants and passengers. Some flight attendants were too ill to【27】______their safety duties, while others have been permanently disabled.
Because of airlines" efforts to【28】______their expenses, cabin-air filters are not cleaned【29】______. The complaints of flight attendants do not always give【30】______to correct maintenance. Airlines turned to recycled air, 【31】______that they would reduce some of their costs. They are not required to put filters in. Airlines are【32】______great pressure to get their flights out【33】______. So they do not pay as much attention to systems that are not as【34】______to flight schedule and safety.
Recent research findings emphasize the concern that filters can【35】______engine chemicals into the cabin air. This may not happen【36】______every flight, but it is a persistent problem.
In a study published in October 1998 an investigation was made【37】______complaints of crew members【38】______air quality and health. More than half of the 200 subjects reported health problems they【39】______to cabin air. It was concluded that these health problems were consistent with【40】______harmful gases and substances.
【21】
A．humid
B．unhealthy
C．unbearable
D．crisp请帮忙给出正确答案和分析，谢谢！