Under Philippine traffic rules, what is the right-of-way rule for vehicles meeting on a narrow mountain road where they cannot pass each other?

1. The vehicle going down the hill must yield and back up, unless the vehicle going up has a clearer view or safety space to yield
2. The vehicle going up the hill must yield and back up under all circumstances
3. The heavier vehicle has the right-of-way
4. The vehicle closest to the cliff wall must yield