Which of the following causes degradation of $RBCs$?

The following water samples are given with their $BOD$ values. Which sample has the lowest level of pollution?
Sample $w - 6 \ mg/L$,Sample $x - 6.5 \ mg/L$
Sample $y - 7 \ mg/L$,Sample $z - 10.3 \ mg/L$

Thermal pollution is more prevalent near

Which of the following intensity of sound becomes a pollutant (in $\text{ dB}$)?

Cotton pollen grains are an important pollutant in which of the following cities?

The dental disease characterized by mottling of teeth due to the presence of a specific chemical in drinking water is caused by which of the following elements?