15. Star X is 200 light-years away from the Earth. Astronauts Joe and Mary, both at age of 30, travel together to star X in a spaceship that moves with constant speed of √0.9999c. When they reach star X, what is the age of Joe observed by Mary? (A) Joe and Mary are already dead since it takes about 200 years to get there. (B) Joe is still about 30 years old since it takes less than 3 months to get there. (C) Joe is about 32 years old. (D) Joe is about 35 years old (E) Joe is 50 years old.