CBSE Questions for Class 12 Medical Biology Environmental Issues Quiz 13 - MCQExams.com

A.Correct option A

B.Explanation for correct option:

• The greater the BOD (Biochemical Oxygen Demand) of the water, the more is its polluting potential because the rapidly the oxygen is depleted in the water. This means lesser oxygen is available for higher forms of aquatic life and thus water will be disturbed. 
• The consequences of the high BOD are the same as those for low dissolved oxygen in water: aquatic organisms become stressed, suffocate and then die because of less availability of oxygen.
• Thus, its polluting potential will be more as the number of decomposing bacteria in water will be high due to which BOD of water increases.

• BOD is the biological oxygen demand. It is the amount of oxygen required by microorganisms in water.
• It is the measure of the amount of organic matter present in the water body. In polluting water, BOD is high as it contains a large number of organisms. 
• Floc is a mesh-like structure formed by the association of bacteria and fungal filaments in sewage treatment. 
• Biological floc is made up of useful aerobic microbes.
• Primary treatment involves the removal of suspended and settled solid waste material. The supernatant acts as effluent. 
• Biogas typically refers to a mixture of different gases produced by the breakdown of much organic matter anaerobically. 
• Biogas is mainly comprised of carbon dioxide, methane, sulfide, and hydrogen. 

• Coal-burning results in the generation of large quantities of residues obtained from coal. 
• These residues contain very fine particles that contribute to the formation of suspended particulate matter (SPM).
• An electrostatic precipitator (ESP) is a filtration method that removes fine particles, like dust and smoke from a flowing gas using the force of an induced electrostatic charge minimally impeding the flow of gases through the unit.

• BOD is Biological Oxygen Demand. It is the amount of oxygen required by microbes to break down the organic matter present in a certain volume of a sample of water. 
• In polluted water, BOD is high as it contains a large number of organisms. 
• High BOD values in the water of a river mean that the water is highly polluted.

Large cities have a big human population and concentration of industries and factories. So, the management of the industrial effluents and the domestic sewage a bif challenge because these cause massive water pollution. Further, the location of industries have to be decided so that the air pollutants emitted from them do not enter the populated areas directly. So, cleanliness and less use of insecticides, proper disposal of organic wastes, sewage and industrial effluents and broader roads and shifting of factories out of residential areas are considered to be essential steps in minimizing the pollution in large cities. Hence, the correct answer is 'All of the above'

Ozone layer is formed in the stratosphere of the atmosphere by the action of UV rays on the oxygen molecules. This ozone layer acts as absorbant for the harmful UV radiations and so, these radiation do not cross the troposphere to reach the earth's surface. However, the ozone depleting pollutants like chlorofluorocarbons reach the stratosphere and break when exposed to the UV rays. This process releases the reactive chlorine which further reacts with ozone to form simple oxygen. That is how the ozone layer is depleted. Hence, the correct answer is 'Chlorofluorocarbons'.