Complete each statement with the correct word or words.

5. Some caves are ____________, while others contain underground streams and lakes.
6. Most ______________of significant size are formed in limestone by the dissolving activity of groundwater.
7. A depression in the ground caused by the collapse of a cave or by the direct dissolution of bedrock by acidic rain or moist soil is a(n) _______________
8. Limestone regions with sinkholes, sinks, and sinking streams are said to have _____________