Answer the following question given this paragraph: 

Placental mammals are therian mammals in which a placenta develops during pregnancy. The placenta sustains the fetus while it grows inside the mother’s uterus. Placental mammals give birth to relatively large and mature infants. Most mammals are placental mammals.


Q: What does the placenta sustain during pregnancy?

 Choices:

- the heart

- the mother

- the endocrine system

- the fetus

A:
the fetus
