Smog is primarily caused by the presence of which of the following?

The scientists who explained that the greenhouse effect and the Earth's atmosphere function in a similar way were ...... .

If water pollution continues at its present rate,it will eventually

The decomposition of organic compounds in the presence of oxygen and without the formation of odoriferous substances is called:

Given below are two statements:
Statement $I:$ Classical smog occurs in cool humid climate. It is a reducing mixture of smoke,fog and sulphur dioxide.
Statement $II:$ Photochemical smog has components,ozone,nitric oxide,acrolein,formaldehyde,$PAN$ etc.
In the light of above statements,choose the most appropriate answer from the options given below:

Which is the greatest source of air pollution these days?