is a Lorentz invariant, meaning that it is the same in any reference frame. We choose as our two events the two flashes of light. We use unprimed variables for the frame of observer and primed variables for the frame of observer . , so implies that
Solving for gives
From the Lorentz transformation equations,
we also have that
Diving the second equation by the first equation gives
Plugging in and gives
Therefore, answer (C) is correct.