Answer:
The <u><em>second </em></u>line of defense.
Explanation:
In our bodies the <em><u>second line of defense</u></em> includes active immunity. It contains non-specific immune responses, but yet still contains specific responses for certain things such as inflammatory and fevers.
B) Oxygen may not have been able to accumulate in the atmosphere
In a red blood cell, the control center is the nucleus. A mitochondria is not a nucleus. It is another organelle that produces energy for the cell.
A mystery organism is non living