一道高考英语的完形填空题 求大神解答

一道高考英语的完形填空题 求大神解答
Every moutain has a peak. Every valley has its low point. Life has its ups and downs, its peaks and its____.
No one is up all the time, nor are they down all the time.
A.points B. summit c.mountains D.valleys

答案选的是D，为什么呢？为什么不是A呢？跪求大神帮忙！