Alligators have always been the most dangerous predators of the Florida Everglades, (31) .................. their long-held position is now being challenged by a growing population...

* Hướng dẫn giải

Alligators have always been the most dangerous predators of the Florida Everglades, (31) but their long-held position is now being challenged by a growing population of Burmese pythons.

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Chọn đáp án A.