The conclusion of the argument is that the decline seen in the United States' overall infant mortality rate does not necessarily indicate "that babies in the United States are now, on average, healthier at birth than they were in the past." We know that a society's infant mortality rate is an "accepted indicator of that society's general health status." So we want to support the claim that just because the infant mortality rates are declining, this does not necessarily mean that, on average, babies in the United States are healthier at birth.
(A) is incorrect because it is irrelevant. The passage states, "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." So, the passage is telling us that even if there are deficiencies being masked in particular localities, the overall rate is still declining. Our conclusion is concerned with babies health ON AVERAGE, not in specific localities. Thus, this answer choice does not do anything to the passage.
(B) is incorrect because, like answer choice (A), it is irrelevant. We want to support the claim that even though the rate of infant mortality in the United States is declining, this does not necessarily mean that, on average, babies in the United States are healthier at birth. Therefore, what contributes to those infants that are dying is irrelevant to the argument.
(C) is CORRECT because it helps explain how the overall infant mortality rate is declining but yet does not mean that, on average, the babies in the United States are healthier at birth. The sophisticated technology has helped saved the lives of premature and low-birth-weight babies that might have contributed to a higher infant mortality rate in the past (thus decreasing the rate of infant mortality). Yet, these babies are still not healthy at birth; they are merely being saved from dying. Therefore, this answer choice helps support the claims made about the implications of the decline in mortality rate.
(D) is incorrect because we are concerned with the health of babies ON AVERAGE. Information about just 11 states does not help or hurt the argument. Answer choice (D) does not help support the claim that the declining infant mortality rate does not necessarily mean that babies are healthier at birth in the United States.
(E) is incorrect because it does not touch upon the health of babies AT BIRTH. A baby gaining weight slowly due to inadequate attention from a caregiver discusses poor health AFTER birth. Therefore, this answer choice is irrelevant.
Hope that was helpful! Let me know if you have any other questions.
