62. Itcan be inferred from the first paragraph that______.
A) watervapor is an air pollutant in localized areas
B) thedefinition of air pollutiom will continue to change
C) a substance becomes an air pollutant only in cities
D) mostair pollutants today can be seen or smelled
63. Inwhat way can natural pollutants play an important role in controlling airpollution?
A) Theyfunction as part of a purification process.
B) Theyare 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.
64.According to the passage, the numerical value of the concentration level of asubstance is only useful if
A) theother substances in the area are known
B) it isa localized area
C) it can be calculated quickly
D) the naturally occurring level is also known
65.Which of the following is best supported by the passage?
A) Toeffectively control pollution, local government should regularly revise the airpollution laws.
B) One of the most important steps in preserving natural lands is to betterenforce air pollution laws.
C) Scientists should be consulted in order to establish uniform limits for allair pollutants.
D) Humanactivities have great impact on air pollution.
66. Thepassage mainly discusses
A ) theeconomic impact on air pollutionC) how much damage air pollutants can cause
B ) what constitutes an air pollutantD) the quantity of compounds added to theatmosphere

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