Assertion. On cooling, the brown colour of nitrogen dioxide disappears.
Reason. On cooling. `NO_(2)` undergoes dimerisation resulting in the pairing of odd electron of `NO_(2)`.
A. If both assertion and reason are true, and is the correct explanation of the assertion.
B. If both assertion and reason are true, but reason is not the correct explanation of the assertion.
C. If assertion is true, but reason is false.
D. If both assertion and reason are false
