MY COUSIN TENDS TOLOOK ON THE BRIGHT SIDEIN ANY CIRCUMSTANCE.A.BE OPT...

Câu 46: My cousin tends tolook on the bright sidein any circumstance.A.be optimistic B.be pessimistic C.be confident D.be smartRead the following passage and mark the letter A, B, C, or D on your answer sheet to indicatethe correct answer .An air pollution is defined as a compound added directly or indirectly by humans to the atmospherein such quantities as to affect humans, animals, vegetation, or materials adversely. Air pollutionrequires a very flexible definition that permits continuous changes. When the first air pollution lawswere established in England in the fourteenth century, air pollutants were limited to compounds thatcould be seen or smelled- a far cry from the extensive list of harmful substances known today. Astechnology has developed and knowledge of health aspects of various chemicals has increased, the listof air pollutants has lengthened. In the future, even water vapor might be considered an air pollutantunder certain conditions.Many of more important air pollutants, such as sulfur oxides, carbon monoxides and nitrogenoxides are found in nature. As the Earth developed, the concentration of these pollutants was altered byvarious chemical reactions; they became components in biogeochemical cycles. These serve as an airpurification scheme by allowing the compounds to move from the air to the water or soil. On a globalbasis, 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 region,human output may be dominant and may temporarily overload the natural purification scheme of thecycles. The result is an concentration of noxious chemicals in the air. The concentrations at which theadverse effects appear will be greater than the concentrations that the pollutants would have in theabsence of human activities. The actual concentration need not be large for a substance to be apollutant; in fact, the numerical value tells us little until we know how much of an increase thisrepresents over the concentration that would occur naturally in the area. For example, sulfur dioxidehas detectable health effects at 0. 08 parts per million (ppm), which is about 400 times its naturallevel. Carbon monoxide, however has a natural level of 0. 1 ppm and is not usually a pollutant until itslevel reaches about 15 ppm.