GMAT Sentence Correction (SC) | Re: Some anthropologists believe that the genetic homogeneity

egmat
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 .

I still dont get why A is not correct ?
"ing" plays 2 role first is describing the action with subject and second it can be an effect of a cause mentioned earlier in the clause.
here why cant "greatly reducing their