Read the following passage and mark the letter A, B, C, or D on your answer sheet to each of the questions
An air pollutant is defined as a compound added directly or indirectly by humans to the atmosphere in such quantities as to affect humans, animals, vegetation, 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 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 more important air pollutants such as sulphur 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 from 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, sulphur dioxide has detectable health effects at 0.08 parts per million (ppm), which is about 400 times its natural level. Carbon monoxide, however, as a natural level of 0.1 ppm and is not usually a pollutant until its level reaches about 15 ppm.
Question: The word "these" in the second paragraph is closest in meaning to ___________.
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A.
the various chemical reactions
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B.
the pollutants from the developing Earth
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C.
the compounds mover to the water or soil
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D.
the components in biogeochemical cycles
Đoạn 2: In the future, even water vapor might be considered an air pollutant under certain conditions. Many of more important air pollutants such as sulphur 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 from human activities.
Tạm dịch: Trong tương lai, ngay cả hơi nước cũng có thể được coi là chất gây ô nhiễm không khí trong một số điều kiện nhất định. Nhiều chất gây ô nhiễm không khí quan trọng hơn như SO2, CO và NO có trong tự nhiên. Khi Trái Đất phát triển, nồng độ của các chất ô nhiễm này đã thay đổi do nhiều phản ứng hóa học khác nhau; chúng trở thành thành phần trong chu trình sinh địa hóa. Chúng đóng vai trò như một chương trình thanh lọc không khí bằng cách cho phép các hợp chất di chuyển từ không khí vào nước hoặc đất trên phạm vi toàn cầu, sản lượng các hợp chất này của tự nhiên làm lu mờ lượng sản phẩm do các hoạt động của con người tạo ra.
Từ "these" trong đoạn thứ hai có nghĩa gần nhất với ___________.
A. các phản ứng hóa học khác nhau
B. các chất ô nhiễm từ Trái đất đang phát triển
C. các hợp chất di chuyển đến nước hoặc đất
D. các thành phần trong chu trình sinh địa hóa
Chọn D
Đáp án : D
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