Questions 18 to 20 are based on the following passage. At the end of the passage, you will be given 15 seconds to answer the questions. Now listen to the passage.

What does the speaker say about ancient Greeks who traveled south

A．They noticed an apparent change in the position of the North Star.
B．They observed eclipses at different rimes of the year.
C．They were the first to estimate the distance between heavenly bodies.
D．They wanted to prove that the Earth was flat.