Which of the following is the most likely explanation for the lack of a filter blocking the passage of alcohol between the maternal and fetal circulations in humans?
A) There has not been enough time to evolve such a barrier.
B) Such a barrier would probably also block important molecules that need to be passed to the fetus.
C) The maternal and fetal blood mix directly together in an area with many villi, so a barrier is impossible.
D) Alcohol has some positive effects on the fetus, so evolution has resulted in an intermediate level of filtering that blocks all but the worst abuses of alcohol.
Answer: B
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