Of the world’s largest AIDS cases countries in 2010, three had the same share of world AIDS patients as they had in 2000. These three countries may serve as examples of countries that succeeded in holding steady their share of the AIDS disease. 

Which of the following, if true, would most seriously undermine the idea that these countries serve as examples as described above ?

If the three countries have different rates of population growth, their share of AIDS cases might appear constant only because the relative proportions of their populations are changing. This would undermine the idea that these countries successfully controlled their share of AIDS cases. Instead, it could mean that demographic changes (population growth) are masking the actual changes in the incidence or prevalence of AIDS.