Which NDB (Non-Directional Beacon) propagation error causes the aircraft's ADF (Automatic Direction Finder) needle to fluctuate or hunt, especially at night near sunset or sunrise?

1. Night effect, caused by the reflection of skywaves off the ionosphere.
2. Coastal refraction, caused by the change in wave velocity as it crosses a shoreline.
3. Terrain effect, caused by reflections from mountainous regions.
4. Quadrantal error, caused by the distortion of the radio wave by the aircraft fuselage.