Final answer:
The build-up of carbon dioxide in the baby's blood triggers a baby to start breathing right after birth.
Step-by-step explanation:
The correct answer is a. The build-up of carbon dioxide in the baby's blood. After birth, the baby's lungs need to expand and start functioning properly in order to take in oxygen and remove carbon dioxide. During pregnancy, the baby receives oxygen through the mother's placenta, so the baby's lungs do not need to be fully functional. However, immediately after birth, the baby's lungs are exposed to air, which causes a change in the baby's blood chemistry. This change triggers the baby's respiratory center in the brain to start sending signals to the muscles involved in breathing, allowing the baby to breathe on their own.
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