In this task, you are given a paragraph, and your job is to generate comma-separated entities present in the given paragraph. Generate entities from a given passage in such a way that (i) they are present in the paragraph, (ii) they are non-duplicate, (iii) they underwent a state change during the process. Avoid creating entities that (i) aren't present in the process paragraph and (ii) aren't significant. 
Q: Dissolution of limestone forms caves. Rain gets carbon dioxide from the air. When rain goes into soil it creates an acid. The acid dissolves the limestone along cracks. Some will dissolve enough to form caves. 
A: 
carbon dioxide ,rain ,acid ,limestone ,caves 