题型：事实信息题

根据题目信息定位到文章的内容：“Methane and ammonia are both … rather than methane.”

B选项跟文章信息相符，“Methane and ammonia are both comparatively fragile molecules that might easily have been broken apart by the ultraviolet sunlight that bathed the young Earth, which had not yet evolved an ozone shield.”

A选项属于无中生有，误解了文章的信息。文章并没有提及小行星撞击“introduced other molecules”。

C选项属于无中生有，误解了文章“reducing atmosphere”的意思。

D选项属于无中生有，误解了文章最后一句话的意思—“rich in carbon dioxide rather than methane”而不是“The carbon dioxide reduced methane and ammonia into simpler substances”。