GRE Reading Comprehension: JiJing 352-GRE阅读机经352篇 - D7JQ2GC5GWQ55XQBJ

Which of the following does the passage identify as a factor affecting the rate at which stars are produced by a galaxy? A. The form of hydrogen contained in the galaxy. B. The density of hydrogen clouds contained in the galaxy. C. The age of the galaxy.