Biological magnification refers to

All statements regarding $BOD$ (Biochemical Oxygen Demand) are correct,except:

$A$: Ecological sanitation is a sustainable system for handling human excreta.
$R$: It is a practical and hygienic method of using dry composting toilets.

One of the chief causative factors of desertification is

Spraying of $DDT$ on crops produces pollution of ...

$A$ large number of fish suddenly die in a lake near a village. Which of the following could be the reason for this?
$(a)$ Excessive use of urea and phosphate fertilizers in nearby fields.
$(b)$ Spraying of $DDT$ in the entire area.
$(c)$ The lake water has turned green and murky.
$(d)$ The population of phytoplankton in the lake has decreased,reducing the photosynthetic capacity.
Which two of the above reasons are responsible?