試題詳解

60. A spacecraft is flying towards Earth at a speed of 0.7 c. When it is 8 light-years away from Earth, it sends a message to Earth at a speed of 0.5 c (relative to the spacecraft). How long will it take for Earth to receive this message?
(A) 5 years
(B) 6 years
(C) 7 years
(D) 8 years
(E) 9 years