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. According to the speaker, what do people often think about astronomers
A．They spend most of their time looking through telescopes. B．They are constantly analyzing data. C．They often live near observatories. D．They devote a lot of time to theoretical problems.