Which of the following is a true statement?

A) Toxic agents that a pregnant woman eats, drinks, or inhales are capable of being transmitted through the placenta.
B) While some substances are allowed to pass through the placenta, viral sized particles, such as the AIDS virus, are effectively blocked.
C) The mother's and the unborn child's blood intermingles in the placenta.
D) The placenta allows only nutritional substances to enter and only waste substances to leave the fetal blood supply.

Toxic agents can cross the placenta and affect the developing fetus, leading to congenital disabilities, developmental issues, and other health problems. This is why pregnant women are advised to avoid substances like alcohol, nicotine, and certain medications. Option B is incorrect because some viruses, like the Zika virus, can cross the placental barrier and affect fetal development. Option C is incorrect because the mother's and baby's blood supplies do not mix in the placenta, but instead exchange gases, nutrients, and wastes through specialized membranes. Option D is incorrect because the placenta also allows certain hormones, antibodies, and other signaling molecules to pass through to the fetus.