49.

Define the following terms:
(i) Source (ii) Sink (iii) Receptor.

(i) Source: The site from which the pollutants or contaminants come is called source. 

(ii) Sink: The material or medium which consumes or interacts with a long-lived pollutant is called sink. A marble deposit acts as a sink for atmospheric sulphuric acid and in the process gets damaged.
 

Oceans are sinks for atmospheric carbon dioxide.

(iii Receptor: Anything that is affected by the pollutant is called receptor. For example, the human being is the receptor of smog as it causes irritation in eyes and breathing problems.