The answer is:
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The hydrosphere.
The country that is an isthmus and links central and Souh America is Panama
Rural settlement means people living in rural areas, that is, not in the cities (which are urban areas).
Less and less people live in rural areas because there are less jobs there than in the city as our farming becomes more and more industrialized - before more people had to work on the farm, as more things had to be done by hand.
