Formation of photochemical smog involves the following reaction in...

When fossil fuels are burnt, a variety of pollutants are emitted into the earth’s troposphere. When these pollutants build up to sufficiently high levels, a chain reaction occurs from their interaction with sunlight in which $NO$ is converted into nitrogen dioxide (${NO}_{2}$). This ${NO}_{2}$ in turn absorbs energy from sunlight and breaks up into nitric oxide and free oxygen atom.

$\begin{cases}{NO}_{2} ( g ) & \overset{hv}{\to} & NO ( g ) & + & O ( g ) \\ & & \left(A\right) & & \left(B\right)\end{cases} - - - - - ( i )$

Oxygen atoms are very reactive and combine with the ${O}_{2}$ in air to produce ozone.

$\begin{cases}O ( g ) & + & {O}_{2} ( g ) & ⇌ & {O}_{3} ( g ) \\ \left(B\right) & & & & \left(C\right)\end{cases} - - - - - ( ii )$

The ozone formed in the above reaction (ii) reacts rapidly with the $NO ( g )$ formed in reaction (i) to regenerate ${NO}_{2}$ . ${NO}_{2}$ is a brown gas and at sufficiently high levels can contribute to haze.

$\begin{cases}NO ( g ) & + & {O}_{3} ( g ) & \to & {NO}_{2} ( g ) & + \\ \left(A\right) & & \left(C\right) & & & \end{cases}$$\begin{array}{ll}{O}_{2} ( g )\end{array}$$- - - - ( iii )$

So, the A, B and C are $NO, O\text{and}{O}_{3}$ respectively.