Blue-baby syndrome is caused by:

Eutrophication is caused by

Which pollutant from motor-car exhaust causes a mental disease?

Which one of the following is an environment-related disorder with the correct main cause?

Photochemical smog does not contain .....

$A$ - Micro-organisms involved in the biodegradation of organic matter in the receiving water body consume a lot of oxygen,and as a result,there is a sharp decline in dissolved oxygen in the stream from the point of sewage discharge.
$R$ - $BOD$ decreases when the dissolved oxygen amount decreases in water.