This is a really bad question. For one thing, "estimated at" is not an idiomatically correct usage. "Estimated to be" is the proper idiom (see OG13 #30). So all of these would be incorrect.

For another, the "correct" answer is not grammatically correct. In B, we would have essentially "the ocean floor... has long been considered too toxic, its vents release poisonous metals." This is a run-on sentence! It has two independent clauses incorrectly joined by a comma.

You're right that D would be preferable, but even D has problems. "Has been considered" and "has" are more parallel, but then it would say "and has hydrothermal vents release...," which makes no sense.

I strongly recommend that you stop studying from this source.

ok folks, the first thing to note here is that this problem is a (poorly written) knock-off of the following GMAT PREP question:
https://www.beatthegmat.com/sc-with-surf ... 43734.html

a couple of things to note:

* apparently (as evidenced by that problem), "estimated at" is ok.
the difference may be what comes next -- og13 #30 follows "at" with something that isn't a noun, whereas the GPREP problem follows it with a noun. (but, then again, it may not be; there's another problem somewhere in the OG that uses "dated at", followed by a non-noun.)

in any case, gmac has distanced itself from testing such idioms (and with good reason), so this isn't worth worrying about.

--

ceilidh.erickson wrote:This is a run-on sentence! It has two independent clauses incorrectly joined by a comma.

yep

I strongly recommend that you stop studying from this source.

i second this notion. when you're dealing with a source that contains errors even when mimicking official problems ... well, as my uncle used to say, that's all kinds of ain't good.