Which of the following is a major factor for desertification?

At the beginning of the twentieth century,forests covered about ....... of the land of India. By the end of the century,it shrunk to.......

According to the $1991$ census,what was the rural population of India (in $\%$)?

$A$ lake near a village suffered heavy mortality of fishes within a few days. Consider the following reasons for this:
$1.$ Lots of urea and phosphate fertilizer were used in the crops in the vicinity.
$2.$ The area was sprayed with $DDT$ by an aircraft.
$3.$ The lake water turned green and stinky.
$4.$ 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?

Assertion: Eutrophication shows an increase in productivity in water.
Reason: With increasing eutrophication,the diversity of the phytoplankton increases.

Jhum cultivation:
$I.$ Also called as slash and burn agriculture,is the farming practice in North Eastern states of India.
$II.$ Farmers cut down the trees of forest and burn the plant remains.
$III.$ The ash is used as a fertilizer and the land is then used for farming or cattle grazing.
$IV.$ After cultivation,the land is left for several years,so as to allow its recovery.
Which of the statements given above are correct about Jhum cultivation?