Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R

Assertion A : Carbon forms two important oxides - CO and CO$$_2$$. CO is neutral whereas CO$$_2$$ is acidic in nature

Reason R : CO$$_2$$ can combine with water in a limited way to form carbonic acid, while CO is sparingly soluble in water

In the light of the above statements, choose the most appropriate answer from the options given below :

'A' in the given reaction is

When the hydrogen ion concentration [H$$^+$$] changes by a factor of 1000, the value of pH of the solution __________

Match List I with List II

Choose the correct answer from the options given below :