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?
There are two opposing viewpoints on the possibility of life existing on other planets.
Despite many decades of searching with telescopes, space probes, and planetary rovers, we have found no evidence of extraterrestrial life.
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 aren’t so sure.
Which is the most effective topic sentence for the paragraph?