Two events occur on the x axis separated in time by ∆t and in space by ∆x. A reference frame, traveling at less than the speed of light, in which the two events occur at the same time:
A. exists no matter what the values of ∆x and ∆t
B. exists only if ∆x/∆tc
D. exists only if ∆x/∆t = c
E. does not exist under any condition
