Concept:Swallow holes and disappearing streams are typical landforms of karst topography, which forms in soluble rock areas.
Explanation:Carbonic acid in rainwater slowly dissolves limestone, creating cracks and openings.
A swallow hole is an opening in the ground where surface water sinks underground.
After sinking, the water flows through caves and channels, appearing as a disappearing stream downstream.
These features develop best where soluble rocks such as limestone are present.
Therefore, they are characteristic of limestone regions, not humid, river-course, or desert regions.
Answer:A. limestone regions