Domestic sewage mainly contains $A$ wastes which are readily decomposed with the help of $B$. Here $A$ and $B$ refer to:

$A$ lake near a village suffered heavy mortality of fishes within a few days. Consider the following reasons for this.
$A.$ Lots of urea and phosphate fertilizer were used in the crops in the vicinity.
$B.$ The area was sprayed with $DDT$ by an aircraft.
$C.$ The lake water turned green and stinky.
$D.$ Phytoplankton populations in the lake declined initially thereby greatly reducing photosynthesis.
Which two of the above were the main causes of fish mortality in the lake?

The pesticide used as a preventive measure in buildings is

At the beginning of the twentieth century,forests covered about ........... of the land area of India. (in $\%$)

Soil conservation is a practice in which soil:

Match the following concentrations of $DDT$ in an aquatic food chain:

Column-$I$	Column-$II$
$(a)$ Small fish	$(1) \;25 \;ppm$
$(b)$ Zooplankton	$(2) \;2 \;ppm$
$(c)$ Fish-eating birds	$(3) \;0.5 \;ppm$
$(d)$ Large fishes	$(4) \;0.04 \;ppm$