Rubio was born in 1971, and his parents were naturalized in 1975.
At the age of 18, Rubio had a choice:
A.) remain a Cuban Citizen
B.) become a U.S. Citizen
When Rubio chose to become a U.S. Citizen, he was then naturalized.
The mere fact he had a decision between two countries is the simplest proof that he was naturalized and not natural born, because natural borns are not forced to make an unnatural choice before becoming natural.
Coincidentally, when Rubio became a U.S. Citizen, he lost his Cuban citizenship, because Cuba doesn't allow Cubans to hold multiple citizenships. (But, I might be wrong about this part.)
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