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  1. #1
    san2612 is offline Member
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    Toefl reading 1997-12

    Please help me with the following reading question
    Questions 41-50
    A useful definition of an air pollutant is a compound added directly or indirectly
    by humans to the atmosphere in such quantities as to affect humans, animals
    vegetations, or materials adversely. Air pollution requires a very flexible definition
    that permits continuous change. When the first air pollution laws were established in
    England in the fourteenth century, air pollutants were limited to compounds that could
    be seen or smelled-a far cry from the extensive list of harmful substances known
    today. As technology has developed and knowledge of the health aspects of various
    chemicals has increased, the list of air pollutants has lengthened. In the future,
    even water vapor might be considered an air pollutant under certain conditions.
    Many of the more important air pollutants, such as sulfur oxides, carbon monoxide,
    and nitrogen oxides, are found in nature. As the Earth developed, the concentrations
    of these pollutants were altered by various chemical reactions; they became components
    in biogeochemical cycle. These serve as an air purification scheme by allowing the
    compounds to move from the air to the water or soil on a global basis, nature's
    output of these compounds dwarfs that resulting form human activities. However, human
    production usually occurs in a localized area, such as a city.

    In this localized regions, human output may be dominant and may temporarily overload
    the natural purification scheme of the cycle. The result is an increased concentration
    of noxious chemicals in the air. The concentrations at which the adverse effects
    appear will be greater than the concentrations that the pollutants would have
    in the absence of human activities. The actual concentration need not be large for a
    substance to be a pollutant; in fact the numerical value tells us little until we know
    how much of an increase this represents over the concentration that would occur
    naturally in the area. For example, sulfur dioxide has detectable health effects at
    0.08 parts per million (ppm), which is about 400 times its natural level. Carbon
    monoxide, however, ahs a natural level of 0.1 ppm and is not usually a pollutant
    until its level reaches about 15 ppm.

    50. Which of the following is best supported by the passage?
    (A) To effectively control pollution local government should regularly review their air pollution laws.
    (B) One of the most important steps in preserving natural lands is to better enforce air pollution laws.
    (C) Scientists should be consulted in order to establish uniform limits for all air pollutants.
    (D) Human activities have been effective in reducing air pollution.
    I chose A but the answer is D. Which one is correct?
    Thanks in advance!

  2. #2
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    Re: Toefl reading 1997-12

    (D) is my choice.
    Questions 41-50
    ...; they became components in biogeochemical cycle[s]. These serve as an air purification scheme by allowing the compounds to move from the air to the water or soil. [O]n a global basis, nature's output of these compounds dwarfs that resulting from human activities.

  3. #3
    san2612 is offline Member
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    Re: Toefl reading 1997-12

    Can you explain it to me? I think human activities have increased air pollution because "human output may be dominant and may temporarily overload the natural purification scheme of the cycle"
    what does "dwarfs" mean in this passage?

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