请教各位一道OG上面的新题---- SC 97

扣肉儿

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 numbersB. 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,

感觉OG上面的解释有点牵强，破折号后面的内容是在解释population bottleneck；破折号本身就有表示解释说明的作用，所以不需要再用其他的词汇来表示解释说明。
A选项中“at some time....an event” 解释了bottle neck怎么发生的，后面的“reducing...”是suffer的结果，如果按Manhattan中所说的就是可以
将前面改成为the sufferance reduced their number。
所以说A是错误出在了哪里？
另外，选项B的结构有点混乱，两个that的作用分别是什么？

期待各位的解答咯~

flonacui

http://www.beatthegmat.com/population-bottleneck-t114262.html