同学你好，该知识点来自沪江网校《1对1资深名师致胜托福【128课时签约直达班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好~

助助这里看了一下C选项how 和explained之间应该是缺了“early astronomers”呢~这是explained是动词过去式。
原文中是说astronomers noticed that at times, several planets seem to stop moving in one direction and start moving backward in their orbits around the Earth, and they came up with a theory....
这题是说通过教授的讲解， 这个理论的出现是由于early astronomers为了解释several planets start moving backward. 所以选C

希望对你有所帮助~