I don't know whether this was one of your options, but South Asia can be very well characterized as a Sub-continent.
In fact, it's is commonly referred to as "the Indian Subcontinent" or even simply "Subcontinent". One of the reason for a distinction of this part of the land is that it lies on a different tectonic plate.
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Answer:
Birth rate.
Explanation:
Countries such as Japan are dealing with a low Birth Rate, meaning the population is aging due to the lack of births to lower the average age.
Physical factors that affect population density include water supply, climate, relief (shape of the land), vegetation, soils and availability of natural resources and energy. Human factors that affect population density include social, political and economic factors.
Answer:
The Atlantic Ocean is the body of water on the east coast of the USA
