On a narrow mountain road where two vehicles cannot pass each other simultaneously and there are no passing bays, which vehicle must normally reverse?

1. The descending vehicle must reverse to let the ascending (uphill) vehicle pass, unless the uphill vehicle is closer to a passing bay
2. The ascending vehicle must always reverse regardless of slope gradient
3. The heavier vehicle must always reverse for lighter passenger cars
4. The vehicle with odd-numbered license plates must yield and reverse