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Soumita Ghosh
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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.













