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**[SOLVED] Annihilation, perhaps a defination problem**

**Problem:**

An electron and an anti-electron undergo mutual annihilation.

Which of the following does not have to be conserved?

A. Electric charge

B. Kinetic energy

C. Lepton number

D. Momentum

**Attempt to question:**

The answer is

**B**but I why not also d. I would consider that the mass of photon is zero. So in both B and D, kinetic energy and momentum are perhaps zero. Problem is: it is impossible to divide by zero. Therefore, this problem seems to a definatition problem. It must be that momentum conserves but why? What is the assumption in the theory?

Thank you for your answers!