Which cities in arabia became part of the islamic empire during the time of muhammad

Some of the cities that conquered during Muhammad's period were Medina, Mecca, Damascus,and Baghdad,
Back then, this city were so flourished and became the center of civilization in the world due to many intellectuals that contributed greatly to the world started appearing from these regions.. By the time of His Death, the Islamic influence has spread to most of the Arabian region.

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The answer to the question is a.