Which oxide of nitrogen is not a common pollutant introduced into the atmosphere both due to natural and human activity?

Which of the following practices will not come under green chemistry?

Acid rain is caused by the presence of $X$ and $Y$ in air. $X, Y$ are respectively

Given below are two statements:
Statement $I:$ Classical smog occurs in cool humid climate. It is a reducing mixture of smoke,fog and sulphur dioxide.
Statement $II:$ Photochemical smog has components,ozone,nitric oxide,acrolein,formaldehyde,$PAN$ etc.
In the light of above statements,choose the most appropriate answer from the options given below:

Acrolein $(X)$ is one of the chemicals formed when $O_3$ and $NO_2$ react with unburnt hydrocarbons present in the polluted air. The structure of '$X$' is

The reagent used in the determination of the chemical oxygen demand $(COD)$ of a water sample is