Smog is essentially caused by the atmospheric presence of:

1. A

   Oxygen and ozone

   (a) Oxygen and ozone. O2 is bulk air; ozone aloft is the shield, at ground it is a pollutant, but the pair is not the classic smog recipe in this key.

2. B

   Ozone and nitrogen

   (b) Ozone and nitrogen. Nitrogen is inert bulk air; this pair does not name the acid-gas smog mix.

3. C

   Oxygen and nitrogen

   (c) Oxygen and nitrogen. Ordinary dry air (~21% O2, 78% N2) is not smog.

4. D

   Oxides of nitrogen and sulphur

   (d) Oxides of nitrogen and sulphur. Textbook smog is tied to NOx (photochemical/Los Angeles) and SOx (reducing/London, coal). (d) Official key.

Summary. Official key is (d). Smog is essentially linked to oxides of nitrogen and sulphur in the air. O2, N2 and ozone as pairs are the distractors.