An Electron-Positron Pair Traveling Towards Each Other with a Speed $\rarr$

Multiple Choice

An electron-positron pair traveling towards each other with a speed of v with respect to their center of mass,collide and annihilate according to e^- + e⁺ $\rarr$ $\gamma$ + $\gamma$ .An observer at rest with respect to the center of mass of the electron-positron pair,detects two photons each of frequency 5.13 * 10²⁰ Hz .What was the speed,v prior to the collision?

A) 0.983 c
B) 0.971 c
C) 0.991 c
D) 0.942 c
E) 0.959 c