A、It is static. B、It is in motion. C、It can’t be measured. D、It’s getting bigger. A短文提到，直到17世纪前，天文学家仍认为宇宙大小是恒定不变的。A中的static“静止的”

Today most astronomers accept the notion that groups of stars that make up the universe are all moving farther and farther away from each other, but until very recently this idea of an expanding universe was not a theory most European scholars believed in since ancient times. And up to about the 17th century most of these scholars thought the size of the universe has remained unchanged since the moment of its creation or perhaps forever, with all the stars remaining more or less in place in relation to each other, but that was challenged in the late 17th century by Issac Newton’s idea of gravity as a force of attraction, which contradicted the idea of a universe that is static, unchanging. If gravity causes all the stars out there in space to attract each other as Newton said, then they couldn’t remain motionless. Sooner or later all the stars will fall in towards each other. Well, scientists then propose a new model. Taking Newton’s theory into account, they didn’t want to abandon the idea of motionless status, but for this model to work, they had to modify Newton’s law of gravity, so they theorized that for distance as large as those between stars, the gravitation force repels rather than attracts. As you might guess, there are other contradictions. But this is pretty resolved by the currently accepted theory, which says the universe is continuously expanding. You’ll be reading all about that as your homework tonight.
16. What does the speaker say about most astronomers today?
17. What did most European scholars believe about the size of the universe before 17th century?
18. What did some scientists try to explain by suggesting that stars repel each other?