The gas that damages the ozone layer is -

1. CFCs 

2. CO₂

3. CH₄ 

4. SO₂

Ozone is a very toxic gas and a strong oxidizing agent. It is still useful for life on earth :

1. because it traps harmful UV-rays.

2. because it balances global warming.

3. because it traps harmful X-rays.

4. because it  traps harmful $\mathrm{\gamma }$-rays.

Ozone depletion occurs in : 

1. Tropospheric pollution.

2. Stratospheric  pollution.

3. Air pollution.

4. Photochemical smog.

Incorrect statement among the following is: 

1. Oxides of sulphur, nitrogen, and carbon are the most widespread air pollutant.

2. pH of drinking water should be between 6.5 and 8.5.

3. Concentration of dissolved oxygen below 6 ppm is good for the growth of fish.

4. Clean water would have a BOD value of less than 5 ppm.