Critical Reasoning (CR) | Re: A society’s infant mortality rate is an accepted indicator of that

A society’s infant mortality rate is an accepted indicator of that society’s general health status. Even though in some localities in the United States the rate is higher than in many developing countries, in the United States overall the rate has been steadily declining. This decline does not necessarily indicate, however, that babies in the United States are now, on the average, healthier at birth than they were in the past.

Which one of the following reasons, if true, most strongly supports