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Questions 61-65 are based on the following passage.
A useful definition of an air pollutantis a compound added directly or indirectly by humans to the atmospherein suchquantities as to affect humans, animals, vegetation, or material adversely (有害地.. Air pollution requires avery flexibledefinition that permits continuous change. When the first air pollution lawswere established inEngland in the fourteenth century, air pollutants werelimited to compounds that could be seen or smelled—a farcry (懸殊的差別. from the extensive list of harmful substances known today. Astechnology has developed andknowledge of the health aspects of variouschemicals has increased, the list of air pollutants has lengthened. Inthefuture, even water vapor (水蒸氣. might be consideredan air pollutant under certain conditions.
Many of the more important airpollutants, such as sulfur oxides, carbon monoxide, and nitrogen oxides,arefound in nature. As the Earth developed, the concentrations ( 濃度. of these pollutants were altered byvariouschemical reactions; they became components in biogeochemical (生物地球化學的. cycles. These serve as an airpurification scheme by allowing thecompounds to move from the air to the water or soil. On a global basis,nature'soutput of these compounds dwarfs that resulting from human activities. However,human productionusually occurs in a localized area, such as a city.
In this localized region, human outputmay be dominant and may temporarily overload the natural purificationscheme ofthe cycles. The result is an increased concentration of noxious ( 有害的. chemicals in the air. Theconcentrations atwhich the adverse effects appear will be greater than the concentrations thatthe pollutants wouldhave in the absence of human activities. The actualconcentration need not be large for a substance to be apollutant; in fact thenumerical value tells us little until we know how much of an increase thisrepresents over the concentration that would occur naturally in the area. Forexample, sulfur dioxide has detectable health effects at0.08 parts per million(ppm., which is about 400 times its natural level.Carbon monoxide, however, has a naturallevel of 0.1 ppm and is not usually apollutant until its level reaches about 15 ppm.
61、It can be inferred from thefirst paragraph that _________
vapor is an air pollutant in localized areas
definition of air pollution will continue to change
C.a substance becomes an air pollutant only in cities
air pollutants today can be seen or smelled
62、In what way can naturalpollutants play an important role in controlling air pollution?
A.They function as part of a purification process.
B.They are dwarfed by the pollutants produced by human activities.
C.They are less harmful to living beings than are other pollutants.
D.They have existed since the Earth developed.
63、According to the passage,the numerical value Of the concentration level of a substance is only useful if
other substances in the area are known
is a localized area
can be calculated quickly
naturally occurring level is also known
64、Which of the following isbest supported by the passage?
A.To effectively control pollution, local government should regularlyrevise the air pollution laws.
B.One of the most important steps in preserving natural lands is tobetter enforce air pollution laws.
C.Scientists should be consulted in order to establish uniform limitsfor all air pollutants.
D.Human activities have great impact on air pollution.
65、The passage mainlydiscusses _________
economic impact on air pollution
The men who race the cars are generally small, with a tight, nervous look. They range from the early 20s to the middle 40s, and it is usually their nerves that go first.
Fear is the driver's constant companion, and tragedy can be just a step behind. Scarcely a man in the 500 does not carry the scars of ancient crashes. The mark of the plastic surgeon is everywhere, and burned skin is common. Sometimes a driver's scars are invisible, part of his heritage. Two young drivers, Billy Vukovich and Gary Bettenhausen, raced in their first 500 in 1968. Less than 20 years before, their fathers also competed against one another on the Indy track—and died there.
All this the drivers accept. Over the years, they have learned to trust their own techniques, reflexes, and courage. They depend, too, on a trusted servant—scientific engineering. Though they may not have had a great deal of schooling (an exception is New Zealand's Bruce McLaren, who has an engineering degree), many drivers are gifted mechanics, with a feeling for their engines that amount to kinship.
A few top drivers have become extremely wealthy, with six-figure incomes from prize money, endorsements, and jobs with auto-product manufacturers. Some have businesses of their own. McLaren designs racing chassis (底盤). Dan Gurney's California factory manufactured the chassis of three of the first four ears in the 1968 Indy 500, including his own second place car.
Yet money is not the only reason why men race cars. Perhaps it isn't even the major reason. Three-time Indy winner(1961, 1964, 1967) A. J. Foyt, for example, can frequently be found competing on dirty tracks in minor-league races, where money, crowds and safety features are limited, and only the danger is not. Why does he do it? Sometimes Foyt answers, "It's in my blood. " Other times he says, "It's good practice. " Now and then he replies, "Don't ask dumb questions. "
26. The statement "it is usually their nerves that go first" means_______.
A. at first they all have a nervous look
B. they often find they can't bear the tension even if they are in good condition
C. someday they find they can't make responses to any risk
D. they can continue their career at most until the middle 40s
27. It can be inferred that a car accident is often coupled with_______.
A. a plastic surgeon B. a companion
C. a risk D. a fire
28. The invisible scars of the drivers mentioned in the second paragraph refers to_______.
A. the regrets left by their fathers B. the fears left by their fathers
C. the cars left by their fathers D. the heritage left by their fathers

29. Bruce McLaren is different from most of the drivers in that_______.
A. he himself designs chassis B. he has an engineering degree
C. he manufactures chassis D. he is a gifted mechanic
30. A. J. Foyt often takes part in minor-league races for_______.
A. prize money B. blood test
C. cheers from the crowd D. enjoyment
【答案解析】
26-30 B D A B D