The reasoning in the argument is flawed because it A. overlooks the number of people in each generation during the last three thousand years who left no descendants B. disregards possible effects of disasters such as famines and plagues on human history C. overestimates the mathematical effect of repeated doublings on population size D. fails to take into account that people now alive have overlapping sets of ancestors E. fails to consider that accurate estimation of the number of people alive three thousand years ago might be impossible