What causes photochemical smog?
a.it is formed by reactions involving sunlight and nitrous oxides and hydrocarbons released from automobile exhaust.
b.it is formed from sulfur oxides and particulate matter released from combustion of combustion of heating oil and coal.
c.it is formed by reactions between cfcs from industrial processes and ice crystals in the upper atmosphere.
d.it is formed when secondary pollutants such as ozone combine with primary pollutants such as hydrocarbons in cold winter temperatures?