As far as we know, Earth is the only planet on which life has evolved, and all known life forms are carbon–based. The...

Skylar on June 6, 2020

@gharibiannick, happy to help!

There are two main differences that make (E) a better answer choice than (C).

First, (E) is making a statement about estimations of probability whereas (C) is making a statement about the restriction of scientific studies. Which of these aligns more closely to our passage? We see that the conclusion of our passage is making a statement about what "our scientific estimates of the probability" should be, which is consistent with answer choice (E).

Secondly, (E) compares estimates "more closely tied to what is known" with estimates "less closely tied to what is known," whereas (C) compares the "observable" to the "unobservable." As above, (E) aligns more closely with the passage. The passage references Earth's environment and capacity for carbon-based life as what is known, as compared to different environments with noncarbon-based life as what is unknown. On the other hand, answer choice (C)'s phrasing of "observable" and "unobservable" creates issues. Do we know that the scientists estimating the probability of extraterrestrial life are able to observe that life? No, so (C) is not as applicable.

Does that make sense? Please let us know if you have any other questions!