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

$A$ 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?

The quantity of $CO_2$ in the atmosphere is about (in $\%$)

Which are the primary constituents of photochemical smog?

In an ecological crisis,whose interference plays an important role?

Where are Ecosan toilets used?