A speed-limit sign shows 70 km/h on a rural road whose unsigned Category B default would be 90 km/h. Which speed must the driver not exceed?

1. 70 km/h
2. The average of 70 and 90, which is 80 km/h
3. 50 km/h, because any sign implies urban rules
4. 90 km/h, because Art. 21 always beats signs