= Solution
{c}
The two <tree-level Feynman diagrams> are the $s$-channel and $u$-channel <quark> exchange graphs, drawn in the middle row of the figure. Along the directed <quark> line the incoming <photon> is absorbed before the outgoing <photon> is emitted in the first graph; those two insertions are reversed in the second. Their internal <four-momenta> are $q_s=p_1+p_2$ and $q_u=p_1-p_4$, respectively. There is no three-photon <Feynman vertex> in ordinary <QED>, and no two-photon-two-quark <contact interaction> in the <Dirac field> <Lagrangian density>.
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