Answer the following question given this paragraph: 

The majority of mammals are placental mammals. These are mammals in which the developing baby is fed through the mother's placenta. Female placental mammals develop a placenta after fertilization. A placenta is a spongy structure that passes oxygen, nutrients, and other useful substances from the mother to the fetus. It also passes carbon dioxide and other wastes from the fetus to the mother. The placenta allows the fetus to grow for a long time within the mother.


Q: What structure passes oxygen and nutrients to a growing fetus in mammals?

 Choices:

- uterus

- spinal cord

- vas deferens

- placenta

A:
placenta
