Sea lions CR

Weighing in at roughly one ton of whiskers, flippers, blubber, and bone each, Stellers are the largest and most northerly of all sea lions. Many spend the summers on Alaska’s Chiswell Island resting, battling, breeding, and feeding. Stellars have suffered a mysterious population decline of about two-thirds since the 1960s, most likely due to pollution, disease, and competition for food from commercial fishing. Stellers should be considered an endangered species and thus be protected.

Which of the following, if true, indicates a flaw in the reasoning above?

A. Stellers are not a popular animal among the zoo-visiting population and, therefore, are not worth saving.
B. It would take too many resources to save animals as big as Stellers.
C. There is no proof that the decline in population among Stellers is due to outside dangers.
D. A species must suffer population declines of 90% or more to be put on the endangered species list.
E. Endangered species sometimes die out anyway.





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[spoiler]Answer: D[/spoiler]

Can anyone explain about the choice C for me? I was looking for an alternative reason to weaken this argument, so if there are other reasons than environmental pollution and feeding, the choice will weaken the conclusion, doesn't it?

Thanks