Question

A particle of mass M disintegrates into two photons. An observer is moving in a direction perpendicular to the photons' momenta, as viewed in the particle's rest frame. In the observer's rest frame, the photons are moving at 90° relative to
one another. Which of the following gives the speed of the observer?
(A) c/4
(B) c/2
(C) c/sqrt(2)
(D) c
(E) sqrt(2) * c