Which of the following best describes the relative orientations of the electric field E, the magnetic field B, and the direction of propagation k for any plane electromagnetic wave traveling in vacuum?
a) E is parallel to k, while B is perpendicular to both
b) E is perpendicular to B, and the propagation vector k lies in the same plane as the two fields
c) All three vectors E, B, and k are mutually perpendicular
d) E and B are parallel to each other and both are perpendicular to k