= Solution
Label the initial species-1 and species-2 <fermions> by $(p_1,s_1)$ and $(p_2,s_2)$, and the final species-1 and species-2 <fermions> by $(p_3,s_3)$ and $(p_4,s_4)$. There is exactly one <tree-level Feynman diagram>: the $t$-channel <scalar propagator> joins the continuous species-1 line $1\to3$ to the continuous species-2 line $2\to4$. Its exchanged <four-momentum> $q_t$ determines $t$, in the notation of <Mandelstam variables>:
$$
\boxed{q_t=p_1-p_3=p_4-p_2,\qquad t=q_t^2.}
$$
The top row of the figure displays this <Feynman diagram>. A crossed attachment $1\to4$ would change species at an <interaction vertex> and is forbidden. There is no initial <antifermion> for an annihilation channel.
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