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.