Which is required for karst topography to form

There are actually 2 requirements for karst topography to form.
Carbonate rock (limestone, and dolomite) and slightly acidic water 
Slightly acidic water will seep into cracks and slowly dissolve the carbonate rock. it will form caverns, sinkholes, disappearing streams and other karst features.

Hope this helped :)

Closing a ring restricts the rotation around individual car bon carbon bonds hope it will you !