TOEFL Listening: ETS-TOEFL听力机经 - 33381BR0U4B3ZNWCP$

Why does the professor mention the discovery of a large dark spot on the surface of Titan? A. To identify one reason scientists now think that it rains on Titan B. To point out the difficulty in distinguishing surface and atmospheric features on Titan C. To emphasize that surface winds have changed the landscape D. To show how lakes are detected on Titan