One Reason We See Relatively Little Genetic Variation Among Human

One reason we see relatively little genetic variation among human regional populations is that:

A) extensive migration among human populations has homogenized gene pools.
B) extensive convergent evolution has homogenized gene pools.
C) genetic drift has reduced variation in most regional populations over time.
D) humans migrated out of Africa so recently that we haven't had enough time for regional variation to accumulate.
E) None of the answer options is correct.