A Subatomic Particle X Spontaneously Decays into Two Particles, a and B

Multiple Choice

A subatomic particle X spontaneously decays into two particles, A and B, each of rest energy 1.40 × 10² MeV. The particles fly off in opposite directions, each with speed 0.827c relative to an inertial reference frame S.
-Which one of the following statements concerning particle X is true?

A) Momentum conservation requires that it was moving in frame S.
B) Energy conservation requires that it must have been at rest in frame S.
C) Momentum conservation requires that it must have been at rest in frame S.
D) Energy conservation requires that its total energy was 280 MeV in frame S.
E) There is not enough information to determine its state of motion at the time of the decay.