The anion/anions that make/makes the water sample unsuitable for drinking, in which ppm level concentration of  ${\mathrm{F}}^{-}=10, {\mathrm{SO}}_4^{2-}=100, {\mathrm{NO}}_3^{-}=50$  is/ are:

Suppose it rains during a thunderstorm. The collected rainwater has a $\mathrm{pH}$ value

Brown haze produced in photochemical smog is due to:

Choose the CORRECT option in the given following statements:

(I) Clean water contains $\mathrm{BOD}$ value more than $5 \mathrm{ppm}$

(II) The soluble ${\mathrm{F}}^{-}$ ion makes the enamel on teeth into much harder as it converts hydroxyapatite into much harder fluorapatite.

(III) When the $\mathrm{pH}$ of the rain water drops below $5.6$, it is called acid rain