Under MUTCD marking rules, why should you not treat a single solid yellow line on a two-way road as ordinary centerline passing guidance?

1. Two-way centerline markings use broken yellow, solid-plus-broken yellow, or double solid yellow patterns; a single solid yellow is not the standard two-way centerline
2. A single solid yellow always means passing is allowed
3. A single solid yellow means the road is always one-way
4. A single solid yellow is used only for pedestrian crossings