Explanation
Correct answer: C.
Choice C is the best answer because it states a conclusion the researchers likely agree with, given the details in the text. The text explains that a biosignature gas is a gas that can be used as an indicator that a planet harbors some form of life and some astronomers have proposed that NH₃ could serve as a biosignature gas. The researchers evaluating this claim found that the atmosphere of rocky planets would be unlikely to reach "detectably high levels" of NH₃ without biological activity, which would support the proposal of NH₃ serving as a biosignature gas. However, the text also states that mini-Neptune planets can produce NH₃ in the absence of biological activity. Thus, the text is structured to lead to the conclusion that detectable levels of NH₃ in the atmospheres of rocky planets could constitute a biosignature, but that is not the case for detectable levels of the gas in the atmospheres of mini-Neptune planets.
Why the other choices are wrong
Choice A
Choice A is incorrect because the text indicates that biological activity likely accounts for detectable levels of NH₃ in the atmospheres of rocky planets but mini-Neptune planets can have detectable levels of NH₃ in their atmospheres in the absence of biological activity. Therefore, both rocky planets and mini-Neptune planets can have detectable levels of atmospheric NH₃.
Choice B
Choice B is incorrect because the text states that for NH₃ to reach detectable levels in the atmospheres of rocky planets likely means they harbor biological activity, meaning that rocky planets with detectable NH₃ usually harbor biological activity. However, that does not entail that every rocky planet with biological activity will have detectable levels of NH₃ in their atmospheres.
Choice D
Choice D is incorrect because the text claims only that some astronomers have proposed using NH₃ as a biosignature gas without mentioning a minimum concentration of atmospheric NH₃ that must be met for it to function as a biosignature gas.