Assertion: The medulla oblongata uses CSF pH as an indicator of blood carbon dioxide concentration.
Reason: Carbon dioxide is the main determinant of the pH of the CSF:

1. Both assertion and reason are true and the reason is the correct explanation of the assertion.
2. Both assertion and reason are true but the reason is not the correct explanation of the assertion.
3. Assertion is true but reason is false.
4. Both assertion and reason are false.