The statement after the question lists the two variables of interest, apparently in the order input, output. The first selection would seem to apply.
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However, there isn't enough information to tell what is important here. It could be that you're interested in making the boat go a particular speed and you want to find the number of rowers necessary to do that. For this scenario, the second selection is appropriate.
(I would suggest you ask your teacher the basis on which you're supposed to tell. Don't take, "I said so" for an answer.)
