Why Is the Cross Section for e- + e+ -> γ Zero?

[jennyjones]

Homework Statement

I am making an old exam of a particle physics course, and i know how to calculate the cross section for example for
bhabha or moller scattering.

now one of the questions on the old exam is:

Explain why e^-+ e⁺ -> γ is zero, but i am not sure why this is, can someone explain this or point me in the right direction.

2.
I would usually when determining the cross section start with drawing the lowest order feynman diagram, my book put t on the x axis, and the determine the vertex and propagator factors, so i can determine the matrix element.

than with the formula dσ/dΩ = 1/64*pi*s * M^2

 

[Orodruin]

Does it perhaps violate some conservation law? Do you even need to draw a Feynman diagram to come to this conclusion?