Which of the following is a secondary pollutant?

A secondary pollutant is formed by chemical reactions that occur in the atmosphere, involving primary pollutants and other atmospheric components. Ozone (O3) is a secondary pollutant that is formed by a
reaction between nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight.
(1) Carbon monoxide (CO) is a primary pollutant that is emitted directly into the atmosphere from sources such
as vehicle exhaust and industrial processes. 
(2) Carbon dioxide (CO2) is also a primary pollutant that is emitted directly into the atmosphere from sources such as fossil fuel combustion and deforestation.
(3) Sulphur dioxide (SO2) is a primary pollutant that is emitted directly into the atmosphere from sources such as coal-fired power plants and industrial processes. However, SO2 can also contribute to the formation of secondary pollutants, such as sulphuric acid (H2SO4), through chemical reactions in the atmosphere