In our solar system only one of the nine planets -- Earth -- qualifies as fit to sustain life. Nonetheless, using this ratio, and considering the astonishingly large number of planetary systems in the universe, we must conclude that the number of planets fit to sustain some form of life is extremely large. The argument is questionable because it presumes which one of the following without providing justification? ["Life similar to Earth's could evolve under conditions very different from those on Earth.", 'If a planet is Earthlike, then life will arise on it.', 'Our solar system is similar to many other planetary systems in the universe.', 'Most other planetary systems in the universe have nine planets.']