A System Consists of Two Photons Moving in Opposite Directions $E$

Multiple Choice

A system consists of two photons moving in opposite directions. One photon has energy $E$ and the other has energy $4 E$ . The total system's mass $M$ is

A) Zero, of course.
B) $3 E$ .
C) 4E.
D) $5 E$ .
E) Something else (specify) .