D. Mediterranean Sea.
Explanation
The Mediterranean Sea is has an area of about 2.5 million km² that is surrounded by the continents of Europe (to the north), Asia (to the east) and Africa (to the south). It is one of the most important in the world due to the fact that great civilizations such as the Greek, Roman, and Egyptian developed on its coasts. On the European coasts of the Mediterranean are the Iberian Peninsula (where Spain is located, where the Spanish language is spoken) and Italic (where Italy is located, where Italian is spoken). In addition, the Mediterranean Sea bathes the coasts of Montpelier, Nice and Marseille, important French cities, where French is spoken. According to the above, the correct answer is D. Mediterranean Sea.
This is true because 6 and 15’s sum is 21.
I'm NOT 100% sure about this answer but I think it's the Lines of Longitude
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