= Solution
Both $t$-channel <Z boson> exchange and $s$-channel annihilation $e^-\bar\nu_e\to W^{-*}\to e^-\bar\nu_e$ occur. Replacing the incident <neutrino> by an <antineutrino> interchanges the two chiral angular weights. In the same contact-interaction regime,
$$
\boxed{\sigma_{\mathrm{iii}}=\sigma_*\left[x^2+\frac{(x+\tfrac12)^2}{3}\right].}
$$
At $x=0.231$ the factor is $0.2315$, between parts (i) and (ii). The comparison is energy dependent outside this regime: near $s=m_W^2$, the annihilation channel has a <W boson> resonance, and the low-<energy> formula is inapplicable.
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