(i) Phosphate fertilisers in water - (d) Eutrophication

Explanation:Phosphate fertilisers in water can enhance algal growth which could cover the water surface and reduces the oxygen concentration in the water. This leads to anaerobic conditions, commonly with accumulation of decay and animal death. This algal bloom inhibits the growth of other living organisms in the water body. This process is called eutrophication.

(ii) Methane in air - (c) Global warming

Explanation:Methane is produced naturally when vegetation is burnt, digested or rotted in the absence of oxygen. Large amounts of methane are released from paddy fields, coal mines and by fossil fuels. It contributes to global warming.

(iii) Synthetic detergents in water - (a) BOD level of water increases

Explanation:Synthetic detergents in water could result in reduction of dissolved oxygen and therefore, increase in BOD level of water. The amount of BOD in the water is a measure of amount of organic material in the water.

(iv) Nitrogen oxides in air – (b) Acid Rain

Explanation:Acid rain is a by-product of a variety of human activities that emit the oxides of sulphur and nitrogen in the atmosphere. Acid rain is harmful for agriculture, trees and plants as it dissolves and washes away nutrients needed for their growth. It causes respiratory ailments in human beings and animals. When acid rain falls and flows as ground water to reach rivers, lakes etc. it affects plants and animal life in aquatic ecosystem.