Final answer:
The downhill-facing vehicle must yield and back up to allow the uphill vehicle to pass, considering it has greater control in this situation.
Step-by-step explanation:
When 2 vehicles meet on a steep road where neither vehicle can pass, the vehicle facing downhill must yield the way. It is common road etiquette that the vehicle going downhill has a greater amount of control when backing up compared to the vehicle going uphill due to the advantage of gravity.
Thus, the correct answer to fill in the blank would be that the vehicle facing downhill must yield and back up until the vehicle going uphill can pass.