D. Oxygen
It refers to the pathway by which a chemical element is moved through both the abiotic and biotic compartment of an ecosystem.
Solution:
nervous system, digestive system, and umbilical cord. A group of tissues in which small blood vessels from the mother and offspring intertwine but do not connect.
This is the required answer.
I think that the answer would be C
It began 3.5 billion years ago
