= Solution
This is the charge-and-parity-conjugate reaction of part (i). There are $t$-channel <Z boson> and crossed <W boson> contributions, with all fermion arrows reversed relative to part (i). For matched unpolarized <kinematics> and negligible <CP violation>,
$$
\boxed{\sigma_{\mathrm{iv}}=\sigma_{\mathrm{i}}.}
$$
Combining all four results in the regime specified in part (i),
$$
\boxed{\sigma_{\mathrm{i}}:\sigma_{\mathrm{ii}}:\sigma_{\mathrm{iii}}:\sigma_{\mathrm{iv}}
\simeq6.12:1:2.57:6.12.}
$$
In particular, conjugating both initial particles restores the part-(i) rate; conjugating the <neutrino> alone does not. Initial <spin polarization> and energies near a weak-boson <mass> require a separate comparison.
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