According to Article 48 of the PRC Road Traffic Safety Law Implementation Regulations, when two heavy vehicles meet on a narrow mountain road where passing is difficult, which vehicle has the right of way?

1. The vehicle driving uphill has priority; however, if the downhill vehicle is already halfway down and the uphill vehicle has not begun climbing, the downhill vehicle passes first
2. The heavier vehicle always has unconditional right of way regardless of slope direction
3. The downhill vehicle always has absolute right of way because it cannot apply brakes
4. The vehicle with the larger engine displacement must always yield to the smaller vehicle