Question

read the following paragraph, which is missing a topic sentence:
according to the “rare 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?
science fiction assumes that the universe bursts with life, but scientists aren’t so sure.
despite many decades of searching with telescopes, space probes, and planetary rovers, we have found no evidence of extraterrestrial life.
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?

Explanation:

The paragraph presents two contrasting ideas - the "rare Earth hypothesis" (unique Earth conditions for life) and the "mediocrity principle" (ordinary Earth, common life elsewhere). A topic sentence should introduce these two opposing viewpoints.

• Option 1 mentions science fiction vs. scientists, which is not the focus of the paragraph.
• Option 2 talks about lack of evidence, which is not the main point (the paragraph is about hypotheses, not evidence - seeking).
• Option 4 is a question about wondering about life, not presenting viewpoints.
• Option 3 directly states there are two opposing viewpoints on life on other planets, which matches the content of the paragraph (the two hypotheses are opposing viewpoints on life's existence elsewhere).

Answer:

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