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

What is the main difficulty in the transition to $CNG$?

Which of the following refers to the increase in the concentration of a toxicant at successive trophic levels?

Read the following statements carefully and select the correct ones.
$I.$ $UV$ rays are essential for the production as well as degradation of ozone gas.
$II.$ Ozone present in the ionosphere acts as a shield absorbing $UV$ radiation coming from the sun.
$III.$ One-fourth of the incoming solar radiation is reflected by the atmospheric gases and clouds,and only half of the incoming solar radiation falls on the earth's surface,heating it. Of this,only a small portion is reflected back.

Assertion : The concentration of methane in the atmosphere has more than doubled in the last $250$ years.
Reason : Wetlands and rice fields are the major sources of methane.

Which of the following changes would be likely to make terrestrial life on this planet impossible?