The depletion of ozone layer was noticed in year 1980.
The large scale destruction of stratospheric ozone first came into focus in 1987 through experiments in antarctica.

 D.O. is dissolved gaseous form of oxygen that is essential for respiration of fish and or aquatic organisms. Wastewater from sewage treatment plants often contains organic materials that are decomposed by microorganisms. The amount of oxygen consumed by se organisms in breaking down waste is known as biochemical oxygen demand or BOD. BOD directly affects amount of dissolved oxygen in rivers and streams. The greater BOD, more rapidly oxygen is depleted in stream, thus reducing oxygen available to higher forms of aquatic life. Pollution, refore, increases BOD of a water body. So, the water pollution status of a water body can be assessed by studying dissolved oxygen (DO) and biological oxygen demand (BOD) of water body. So, correct option is 'DO and BOD'.

Biochemical oxygen demand is a measure of how much dissolved oxygen is being consumed as microbes break down organic matter. A high demand, refore, can indicate that levels of dissolved oxygen are falling, with potentially dangerous implications for river's biodiversity. High biochemical oxygen demand can be caused by: high levels of organic pollution, caused usually by poorly treated waste water; high nitrate levels, which trigger high plant growth. Both result in higher amounts of organic matter in river. When this matter decays, microbiological activity uses up oxygen.

• Deforestation is process of excessive cutting of trees which adversely affects environment.
• The trees maintain balance of oxygen and carbon dioxide in atmosphere. Deforestation leads to an increase in carbon dioxide concentration which causes global warming and poses a major threat to environment. 
• Shifting of climate and disturbance in biodiversity ratio is or major effect of deforestation.

Dissolved oxygen (DO) refers to amount of oxygen dissolved in water. It depends on following factors:

The correct answer is Stratosphere. Option C

BOD stands for Biological Oxygen Demand. It is  amount of dissolved oxygen required by aerobic organisms to break down organic material present in water. It indicates amount of organic pollution present in an aquatic ecosystem. It is also used in wastewater treatment to decompose organic wastes by aerobic organisms.

The destruction of ozone layer present in stratosphere. The main cause of ozone depletion is CFC gas. When UV rays from sun light breaks CFC molecule and releases chlorine. These chlorine molecule n split ozone molecule into  $O_3$ and free oxygen.

The depletion potential is roughly equivalent, nitrous oxide could have a more damaging effect because it is much more abundant. Global human emissions of N2O are roughly 10 million tonnes per year, compared to slightly more than 1 million tonnes from all CFCs at peak of ir emissions. The CFCs and N2O are very stable in troposphere, where y are emitted and are transported to stratosphere where y release active chemicals that destroy stratospheric ozone through chlorine- or nitrogen oxide?catalyzed processes.