OG17-736 句子结构的问题，求解答

May97

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 enetic 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,

OG对C的解释：The modifier of event is expressed with a wordy passive construction, which destroys the parallelism between it and what follows.
这里应该是说our enetic variation and reduced their numbers should be parallel，但是即使是B选项似乎也没有be parallel

实在不明白and thus our enetic variation是一个什么结构。。。

May97

还有OG对A选项的解释： The agent or cause of reducing is unclear.
可是我觉得这里reducing their number是可以看成main clause的 IMMEDIATE CONSEQUENCE

Power2Bowen

我是这样理解的A选项的，greatly reducing的逻辑主语是前面的ancestor。那么形成的逻辑上的错误。并不是ancestor导致了greatly reducing。