Question

The lower boundary of ______us considered to be the boundary of the continents
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Answer 1

Boundaries between the continents of Earth
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The boundaries between the continents of Earth are generally a matter of geographical convention. Several slightly different conventions are in use. The number of continents is most commonly considered seven but may range as low as four when the Americas and Afro-Eurasia are each considered a single continent. An island can be continental, and therefore associated with the continent on the same continental plate, or oceanic and associated with a continent or geographic region by convention.


Color-coded map of continents:
Americas
North America
South America
Afro-Eurasia
Eurasia
Europe
Asia
Africa
Australia
Antarctica

Map of island countries: these states are not located on any continent-sized landmass, but they are usually grouped geographically with a neighbouring continent
There are three overland boundaries subject to definition:

between Europe and Asia (dividing Eurasia): along the Turkish Straits, the Caucasus, and the Urals and the Ural River (historically also north of the Caucasus, along the Kuma–Manych Depression or along the Don River)
between Asia and Africa (dividing Afro-Eurasia into Africa and Eurasia): at the Isthmus of Suez
between North America and South America (dividing the Americas): at some point on the Isthmus of Panama, with the most common demarcation in atlases and other sources following the Darién Mountains watershed that divides along the Colombia–Panama border where the isthmus meets the South American continent (see Darién Gap)