Can you explain variant D?

why cant it be option c

Every human being who has ever lived had two parents.

All of us has mom and dad

Therefore, more people were alive three thousand years ago than are alive now.

More people were alive than now

(A) overlooks the number of people in each generation during the last three thousand years who left no descendants

we do not know anything about because we do not have information

(B) disregards possible effects of disasters such as famines and plagues on human history

disasters are not mentioned. We do not know

(C) overestimates the mathematical effect of repeated doublings on population size

these math calculations should have a back up in their data

(D) fails to take into account that people now alive have overlapping sets of ancestors

yes this is true because people having overlap could be the case that 3k years ago maybe there were not more people than now

(E) fails to consider that accurate estimation of the number of people alive three thousand years ago might be impossible

we do not know