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