Under Article 81 of the Implementation Regulations, when visibility on an expressway is reduced to less than 100 meters (能见度小于100米) due to dense fog or torrential rain, what statutory limits apply to a passenger bus?

1. Speed must not exceed 60 km/h, and following distance must be at least 100 meters
2. Speed must not exceed 20 km/h, and must stop immediately in the travel lane
3. Turn on fog lights, low beams, clearance lamps, position lamps, and hazard flashers; speed must not exceed 40 km/h, and following distance must be at least 50 meters
4. Maintain 50 km/h with high beams illuminated