(i) Public transport in Delhi uses CNG since 2002. List the advantages of this fuel policy.
(ii) BOD was measured in two different places A and B of a river in the direction of its flow. BOD value was higher at A than B. What do you infer from this observation and why ?

(i) Delhi Government shifted to CNG (Compressed Natural Gas) because :
(a) It burns more efficiently, unlike petrol or diesel, in automobiles and very little of it is left unburnt.
(b) CNG is cheaper than petrol and diesel.
(c) It cannot be siphoned off by thieves.
(d) It cannot be adulterated like petrol or diesel.
(ii) Biochemical oxygen demand (BOD) refers to the amount of the oxygen that would be consumed if all the organic matter in one liter of water were oxidized by bacteria.
As the BOD value is higher at A, therefore, the amount of dissolved oxygen is lower in that place.
Excessive growth of algae, plants and animals in water bodies due to the nutrient enrichment, particularly with nitrogen and phosphorus, and discharge of domestic sewage into a river results in the rise of BOD. However, as the organic matter is decomposed, there is a gradual rise in the amount of dissolved oxygen downstream.