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?
A: There is at least one moon that orbits Alpha.
B: Every moon in S4 orbits the same planet.
C: Every planet in S4 is orbited by more than one moon
D: Alpha is the only planet in S4.
Choose A, B, C or D as your solution.
D