Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions from 43 to 50.
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 changes. 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 more important air pollutants, such as sulfur oxides, carbon monoxides and nitrogen oxides are found in nature. As the Earth developed, the concentration of these pollutants was altered by various chemical reactions; they became components in biogeochemical cycles. 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 such a region, human output may be dominant and may temporarily overload the natural purification scheme of the cycles. The result is an 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 has a natural level of 0. 1 ppm and is not usually a pollutant until its level reaches about 15 ppm.
For which of the following reasons can natural pollutants play an important role in controlling air pollution?
A. They're less harmful to living beings than other pollutants.
B. They function as part of a purification process.
C.They occur in greater quantities than other pollutants.
D. They have existed since the Earth developed.
Kiến thức: Đọc hiểu
Giải thích:
Vì lý do nào sau đây, các chất gây ô nhiễm tự nhiên có vai trò quan trọng trong việc kiểm soát ô nhiễm không khí?
A. Chúng ít gây hại cho cuộc sống hơn các chất gây ô nhiễm khác.
B. Chúng hoạt động như một phần của quá trình thanh lọc.
C. Chúng xuất hiện với số lượng lớn hơn các chất gây ô nhiễm khác.
D. Chúng đã tồn tại kể từ khi Trái đất phát triển.
Dẫn chứng: Many of more important air pollutants, such as sulfur oxides, carbon monoxides and nitrogen oxides are found in nature. As the Earth developed, the concentration of these pollutants was altered by various chemical reactions; they became components in biogeochemical cycles. These serve as an air purification scheme by allowing the compounds to move from the air to the water or soil
Tạm dịch: Nhiều chất ô nhiễm không khí quan trọng hơn, chẳng hạn như oxit lưu huỳnh, carbon monoxit và nitơ oxit được tìm thấy trong tự nhiên. Khi Trái đất phát triển, nồng độ của các chất gây ô nhiễm này đã bị thay đổi bởi các phản ứng hóa học khác nhau; chúng trở thành các thành phần trong chu kỳ sinh hóa. Chúng phục vụ như là một chương trình lọc không khí bằng cách cho phép các hợp chất di chuyển từ không khí đến nước hoặc đất.
Đáp án: B