18. As a spaceship heads directly to Earth at a velocity of 0.8c, it sends a radio signal to Earth. When those radio waves arrive on Earth, their velocity relative to Earth is
(A) c
(B) 0.8c
(C) 1.8c
(D), where v_E is the velocity of the Earth
(E), where v_E is the velocity of the Earth

考試重點本題測驗的是愛因斯坦特殊相對論...