TOEFL Reading: ETS-TOEFL阅读机经 - QL38WMOUG75B47C7L$

Paragraph 6 answers which of the following questions about quasars? A. What is the ratio of the number of quasars to the number of spiral galaxies? B. Why was the discovery of quasars of importance for cosmologists? C. Why were quasars not discovered before the 1960s? D. How do quasars produce so much light?