$C{H_3}C{H_2}OH\xrightarrow{{P + {I_2}}}A\xrightarrow[{ether}]{{Mg}} $ $B\xrightarrow{{HCHO}}C\xrightarrow{{{H_2}O}}D$
\((2)\) \(\left(C H_{3}\right)_{3}-C-O H \frac{20^{\prime \prime} \cdot H_{2} S O_{4}}{363 \,K}\) \(\left(C H_{3}\right)_{2}-C=C H_{2}+H_{2} O\)