What is the two-second following distance rule, and when should a California instructor teach students to increase it?

1. Maintain a 2-second gap at all times with no exceptions
2. Maintain at least a 2-second gap, increasing to 3-4 seconds in adverse conditions such as rain, fog, or heavy traffic
3. The 2-second rule only applies on freeways
4. Maintain a 2-second gap only when following large trucks