Population density is the number of people per unit of area. Usually measured by how many people are in a square mile or square kilometer.
I think the answer is "State", you probably should quote me on this though...
Researchers found a total of 138 volcanoes in the West Antarctic Ice Sheet overlying the West Antarctic Rift System, 91 of which had never before been identified. Basically they have never seen those types of volcanoes before.
Answer:
b
Explanation:
advances made in metallurgy