Twin sisters become astronauts. One sister goes on a space mission lasting several decades while the other remains behind on Earth. Which of the following statements concerning their relative ages is true?

(a) The sister who was on the mission in space is older than her twin once they reunite on Earth.
(b) The sister who remained on Earth is older than her traveling twin once they are reunited on Earth.
(c) The sisters are the same age when the traveling twin returns to Earth because each sister was traveling at the same speed relative to the other as measured in each other's reference frames.
(d) This is a paradox so there is no possibility of comparing their ages.