= Solution
There are two <tree-level Feynman diagrams> for <Compton scattering>: an $s$-channel ordering in which the electron first absorbs the incoming photon, with internal momentum $p+q$, and a crossed $u$-channel ordering in which it first emits the outgoing photon, with internal momentum $p-q'$.
\Image[/media/past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2019/iii/paper-301-compton-diagrams.png]
{title=Tree-level Compton-scattering diagrams}
{description=The two orderings of photon absorption and emission give the electron-exchange s-channel and u-channel diagrams.}
{height=430}
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