= Solution
For B, $\Delta V^\ddagger=+12\,\mathrm{cm^3\,mol^{-1}}$ is consistent with bond fission and release/reorganization of electrostricted solvent on reaching a fragmentation transition state. The ordinary methoxy <ester> has $\Delta V^\ddagger=-13\,\mathrm{cm^3\,mol^{-1}}$, consistent with associative <hydroxide> addition and the compact, strongly solvated developing tetrahedral species.
The opposite signs corroborate distinct mechanisms. Since both values include solvation and any pre-equilibria, they should not be interpreted merely as geometric volumes of isolated molecules.
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