Explanation
Correct answer: C.
Choice C is the best answer. "Still" logically signals that the information in this sentence about Old Faithful’s thriving bacteria species is true despite the previous claim that conditions at the geyser seem as if they would be inhospitable to life.
Why the other choices are wrong
Choice A
Choice A is incorrect because "thus" illogically signals that the information in this sentence is a result or consequence of the previous claim about Old Faithful’s seemingly inhospitable conditions. Instead, this information about the geyser’s many thriving bacteria species is true despite the previous claim.
Choice B
Choice B is incorrect because "specifically" illogically signals that the information in this sentence provides specific, precise details elaborating on the previous claim about Old Faithful’s seemingly inhospitable conditions. Instead, this information about the geyser’s many thriving bacteria species is true despite the previous claim.
Choice D
Choice D is incorrect because "in other words" illogically signals that the information in this sentence serves as a paraphrase or restatement of the previous claim about Old Faithful’s seemingly inhospitable conditions. Instead, this information about the geyser’s many thriving bacteria species is true despite the previous claim.