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 the claim made above about the implications of the decline? ['The United States has been developing and has achieved extremely sophisticated technology for saving premature and low-birth-weight babies, most of whom require extended hospital stays.', 'Low birth weight is a contributing factor in more than half of the infant deaths in the United States.', 'Babies who do not receive adequate attention from a caregiver fail to thrive and so they gain weight slowly.', 'In eleven states of the United States, the infant mortality rate declined last year.']