no brain er (B)

I think the clue to answer this question is given by the fact that the argument says that manatees like to dwindle in waters where "Americans like to play". That clearly links the answer to the use of motorboat, which can inadvertitely cause troubles to the poor animals.

Answer is B only, as it points out that some of them are getting killed un-intensonaly ...

BTW, if some mgmat representative is hearing this, this question is a ditto copy from some of the new gmatprep questions, where the dolphins are replaced by elephants. You poor chaps for $$ dont do these cheap stuff. Do any one wanna see gmatprep Q's..

IMO C

IMO B. "None of these" clearly says all the animals are in danger.

One more vote for C

I think the answer should be A.

Because it says that despite various effort, none of these animals can be considered safe. Which means, there is some unknown factor affecting the population and A talks about the mysterious killing of manatees.

Opening this thread again, came across this question in the test and the explanation was not at all convincing.

OA B

I also chose A when I took the test and I also not convince with the explanation.
The stimulus clearly says

Despite conservation efforts, criminal penalties for harming these creatures, and an overabundance of SAVE THE MANATEE! bumper stickers, none of these animals can be considered safe.

B) All manatees swim at depths that make them vulnerable to the blades of motorboat engines

I thought B couldn't be the answer since motorboat are driven by people who can read the bumper stickers and stop driving the motorboats ....

I also think that the right answer is A.