The correct option is D Air
Smog is the secondary air pollutant. Smog is opaque or dark fog having condensed water vapours, dust, smoke and gases like sulphur dioxide, hydrogen sulphide, nitrogen oxides etc. Smog can be classical or photochemical. Classical smog has the reducing environment. Photochemical smog is yellowish brown opaque smog having an oxidising environment with a little smoke. Photochemical smog contains secondary pollutants of photochemical oxidants.