Which one of the following statements is not valid for aerosols?

$BOD$ of waste water is estimated by measuring the amount of

$A$ - 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.

Select the correct option for catalytic converters.

Assertion $(A)$: Pollution is always caused by human activities.
Reason $(R)$: Pollution is not different from contamination.

Drinking of mineral water with a very low level of pesticides (about $0.02 \text{ ppm}$) for long periods may: