In the Medical Diagnostic Technique Known as Positron Emission Tomography $\gamma$

Multiple Choice

In the medical diagnostic technique known as positron emission tomography (PET) , a positron and an electron annihilate each other and two $\gamma$ -ray photons are emitted. What is the angle between the momentum vectors of the two photons?

A) zero degrees
B) 45°
C) 90°
D) 180°
E) Any angle is possible.