TOEFL Listening: TPO-TOEFL听力TPO - X0ER7P0Z3V8348RDR$

What is the main purpose of the lecture? A. To explain the various ways that sulfuric acid is involved in the formation of caves. B. To describe caves and other geologic formations in U.S. national parks. C. To use Lechuguilla Cave as an example of how most caves form. D. To discuss the formation and characteristics of an unusual type of cave.