Question

read the following paragraph, which is missing a topic sentence: according to the
are earth hypothesis,\ the conditions that support life on earth are unique, and the odds of similar conditions being found on another planet are close to zero. in contrast, the \mediocrity principle\ holds that our planet, solar system, and galaxy are utterly ordinary, making it likely that life is common elsewhere. which is the most effective topic sentence for the paragraph? there are two opposing viewpoints on the possibility of life existing on other planets. have you ever looked up at the stars and wondered if anyone was out there looking back? science fiction assumes that the universe bursts with life, but scientists arent so sure. despite many decades of searching with telescopes, space probes, and planetary rovers, we have found no evidence of extraterrestrial life.

Explanation:

The paragraph discusses two opposing viewpoints on extraterrestrial life: "rare Earth hypothesis" (unique Earth conditions) and "mediocrity principle" (life is common). The option "There are two opposing viewpoints on the possibility of life existing on other planets" introduces these two viewpoints, which the rest of the paragraph elaborates. Other options either ask a question (not a topic sentence) or talk about science fiction/ search efforts, which are not the main focus of the paragraph's discussion on the two hypotheses.

Answer:

There are two opposing viewpoints on the possibility of life existing on other planets.