GMAT Critical Reasoning (CR) | A new species of fish has just been discovered living in gre

Answer is D, as among all the options only D can be proved from the argument.
D. The waters off Papua New Guinea are generally hospitable to eels.
>>"Because of the habitat where it was found and its obviously eel-like body, almost all scientists believe that the fish is an eel. "

Why C is wrong:
C. Some fish with cartilaginous skeletons are not sharks.
>>scientists are still unsure as to the fish's precise classification, but they agreed immediately that the most logical
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