The hypothesis we're trying to strengthen is:
"Most marine life can only thrive in areas with rugged terrain that affords them protection from predators."
We're trying to say that the reason we find marine life near these vents is because of the protection of the rugged terrain.
B talks about warmth and nutrients, which is part of the wrong hypothesis -- it makes it sound like we're not sure whether the warmth and nutrients cause the marine life to be found there, or whether the rugged terrain causes marine life to be there.
In A, on the other hand, we say that the vents near which we find marine life effectively cause rugged terrain... so we like this one better, since it states that the rugged terrain is key without adding confusion about warmth and nutrients.
Make any more sense? This one is a little tricky.