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 even 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 even so as to reduce their numbers greatly

OA:B

i only crossed out A and E.
D is lack of "THAT" so i didn't choose it.

thank u.