The effect of pollution is first marked on:

Which of the following is a source of nitrogen oxides $(NO_x)$?

What is $BOD$?

$A$ - 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.

If the concentration of lead in the blood reaches ........,it is considered a warning sign.

Choose the correct option for catalytic converters.