CBSE Class 9 Answered

(3) X- Nitrogen compounds, Y- Oxygen

The sewage may contain organic waste from the animals(sewage added) which when washes into the waterways(downstream). This sudden introduction of organic material increases the amount of nitrogen (basically nitrites and nitrates from the wastewater) in the water that leads to eutrophication. 

Organic pollution occurs when an excess of organic matter, such as manure or sewage, enters the water. When organic matter increases in a pond, the number of decomposers will increase. These decomposers grow rapidly and use a great deal of oxygen during their growth. This leads to a depletion of oxygen as the decomposition process occurs. A lack of oxygen can kill aquatic organisms. As the aquatic organisms die, they are broken down by decomposers which leads to further depletion of the oxygen levels.