IN WHICH AREA HAVE AMERICANS RECEIVED THE LEAST AWARDS

Câu 54. In which area have Americans received the least awards?

A Economics B Science C Peace D Literature

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.

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 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 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 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, has a natural level of 0.1 ppm

and is not usually a pollutant until its level reaches about 15 ppm.