Which, of the following, gases contributes  nearly 50 % to the green house effect (which, in turn, causes global warming) ?

1. SO₂

2. SO₃

3. CO₂

4. Water vapour

The oxygen present today in the atmosphere- 

1. Is a plant product.

2. Came from ozone.

3. Was present in the beginning.

4. Is produced by carbon dioxide.