参考答案

正确答案：C
解析：本题为细节题，关键在于本文的最后一句“For now, scientists accept the impact theory because it explains what we know today about the earth and the moon．”。本句说明这一理论合理，原因在于选项C．it can answer many questions raised about the moon and the earth。选项A中“what happened billions of years ago”尚未被科学家发现，科学家只知道两个球体的年龄为billions of years。选项B中“the newly gathered information about Mars”文章中没有提及，因此不是本文的答案。选项D有两处不对，一是“impact theory”在原文中的句子主体是“scientists”，而非“astronauts”；二是地球和月球也并非同样古老(“of the same age”)。