At one time, many astronomers assumed that Earth remains motionless while the stars revolve around it. They concluded from this that the stars were not more than a few million miles from Earth. They reasoned that if the stars were farther away, they would have to move at tremendously great speeds in order to circle Earth during the day and reappear in roughly the same positions each night. Which one of the following is an assumption required by the reasoning described above? ['All stars move at exactly the same speed when they are revolving around Earth.', 'Earth does not remain motionless while the stars revolve around it.', 'Stars do not move at tremendously great speeds.', 'If the stars do not revolve around Earth, it is possible for at least some stars to be more than a few million miles from Earth.']