Astronaut: Any moon, by definition, orbits some planet in a solar system. So, the moons in solar system S4 all orbit the planet Alpha. The astronaut's conclusion follows logically if which one of the following is assumed? ['There is at least one moon that orbits Alpha.', 'Every moon in S4 orbits the same planet.', 'Every planet in S4 is orbited by more than one moon', 'Alpha is the only planet in S4.']