Laser light travels through the arms of the detector at exactly 299,792,458 m/s which is the official speed of light. How do we know that the light in the detector can travel at exactly that speed?

a. A vacuum exists within the pipes of the arms so that no air interferes.

b. All electromagnetic radiation (light) travels at that speed, always. 

c. Laser light is powerful enough to cut through and not be disturbed by air molecules. 

d. Red light always travels at 299,792,458 m/s regardless of the medium.