TOEFL Listening: ETS-TOEFL听力机经 - 6OZCS132QUONA1HEP$

What are two indirect methods that astronomers have used to detect exoplanets? A. They look for rings surrounding distant light sources. B. They look for the periodic reduction of light from a star. C. They look for the presence of other known exoplanets in the same star system. D. They look for slight movements of a star.