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 sometime 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,


The sentence says that- Some anthropologists believe that the genetic homogeneity is the result of a “population bottleneck”-- 

The second part explains “population bottleneck”

The next clue to solving this question is - what greatly reduced their numbers? An event. Therefore we need the relative pronoun “that” to modify events.

This leaves us with option B.


Ancestors did not suffer an event in order to (intention) reduce their events. This eliminates options C and E.

The participial modifier- greatly reducing modifies the preceding clause and not “event”. 

B is the answer.