[SC悬赏令] SC求解答！破折号用法题

Some anthropologists believe that thegenetic homogeneity evident in the world’s people is the result of a"population bottleneck" -at some time in the past our ancestorssuffered an event, greatly reducing their numbers and thus our geneticvariation.

A at some time in the past ourancestors suffered an event, greatly reducing their numbers  

B that at some time in the past our ancestors suffered an event thatgreatly reduced their numbers

C that some time in the past our ancestors suffered an event so thattheir numbers were greatly reduced,  

D some time in the past our ancestors suffered an event from whichtheir numbers were greatly reduced  

E some time in the past, that our ancestors suffered an event so as toreduce their numbers greatly,

答案选B。想问：1. population bottleneck表示人口瓶颈，是指一个时期吗？为什么一定要加at?如果是同位语从句为什么不是some time in the past...修饰抽象名词
population bottleneck？

2. 为什么that要补出？貌似很多同位语前面都没有加that呀，还是说同位语结构可以不加that,但是同位语从句一定要加？

望各位大神赐教！谢谢！