OG P694 Q737 一个小小的问题~

小陈冲鸭

737. Some anthropologists believe that the genetic homogeneity evident in the world's people is the result of a "population bottleneck"—at some time in the past our ancestors suffered an event, greatly reducing their numbers and thus our genetic variation.
(A) at some time in the past our ancestors suffered an event, greatly reducing their numbers
(B) that at some time in the past our ancestors suffered an event that greatly reduced their numbers
(C) that some time in the past our ancestors suffered an event so that their numbers were greatly reduced,
(D) some time in the past our ancestors suffered an event from which their numbers were greatly reduced
(E) some time in the past, that our ancestors suffered an event so as to reduce their numbers greatly,
答案选B
代入B，整句话就是Some anthropologists believe that the genetic homogeneity evident in the world's people is the result of a "population bottleneck"— that at some time in the past our ancestors suffered an event that greatly reduced their numbers and thus our genetic variation.
我的问题是，倒数第五个词and前面的成分根本不平行，前半部分是句子，后半部分是名词，怎么会这样？

小陈冲鸭

我傻了！就是their numbers