In positron emission tomography (PET), a positron is created by the decay of a nucleus attached to a glucose molecule. This positron then annihilates an electron (because the positron is antimatter; that is, it is an anti-electron). The annihilation reaction produces two gamma rays, traveling off in opposite directions. When the gamma rays are detected, their origin in the body reveals how and where glucose is absorbed. 

(a) Find the energy, in MeV, of one of these gamma rays. 

(b) Find the frequency of the gamma ray in part (a). 

(c) Explain why there are two gamma rays emitted in opposite directions. Hint: A positron has the same mass as an electron.