Answers 1 and 2 are cases where the speed must be reduced as prescribed in Article 5 of Circular No. 13/2009/TT-BGTVT
Answer 3 is wrong because the vehicle being overtaken must slow down,, not is the vehicle passing another vehicle.
When there are speed limit signs or obstacles on the road; when visibility is limited; when passing at an intersection, where a road intersects with a railway, roundabout , rough road, slippery, dusty.
When crossing the bridge, the culvert is narrow; when going up near the top of the slope, when going downhill, when going through schools, crowded places, densely populated places, with houses near the road.
When driving a vehicle overtaking another vehicle on a national highway.