Which of the following molecules is present in photochemical smog?

What percentage of the total atmosphere is occupied by the homosphere?

The living organisms or the surroundings of the earth's composition are known as .....

What is the percentage contribution of $CO$ to the total atmospheric concentration due to forest fires?

The concentration of dissolved oxygen in water for the growth of fish should be more than $X \ ppm$ and the Biochemical Oxygen Demand $(BOD)$ in clean water should be less than $Y \ ppm$. $X$ and $Y$ in $ppm$ are,respectively:

The measured $BOD$ values for four different water samples $(A - D)$ are as follows:
$A = 3 \, ppm; B = 18 \, ppm; C = 21 \, ppm; D = 4 \, ppm.$
The water samples which can be called as highly polluted with organic wastes,are .... .