b
$\begin{matrix}
O\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, \\
||\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, \\
Et\,-C\,-C{{H}_{3}}+C{{H}_{3}}C{{H}_{2}}MgI\xrightarrow[(2){{H}^{\oplus }}]{} \\
\end{matrix}\underset{\begin{smallmatrix}
{{3}^{o}}\,alcohol \\
(cannot\,be\,oxidized)
\end{smallmatrix}}{\mathop{\begin{matrix}
OH \\
|\,\,\,\,\,\, \\
Et-C-C{{H}_{3}} \\
|\,\,\,\,\, \\
Et\,\,\,\, \\
\end{matrix}}}\,$