Which of the following gases,when passed through a warm dilute solution of $H_2SO_4$ in the presence of $HgSO_4$,gives acetaldehyde?

Hydration of propyne results in the formation of.......

To convert $1$-butyne to $1-D$-butanal,one would carry out the following steps :
$(I)$ Sodium amide,then $D_2O$
$(II)$ Disiamylborane,then hydrogen peroxide/sodium hydroxide
$(III)$ The transformation can not be carried out with the indicated reagents.

When $CH_3CH_2CH_2CHCl_2$ is treated with $2 \ g$ equivalent of $NaNH_2$,the product formed is:

$C_3H_4 + H_2O \xrightarrow[333 \ K]{Hg^{2+} / H^{+}} [X] \rightleftharpoons{\text{Tautomerisation}} Y$
$X$ and $Y$ are respectively

The synthesis of $3-$octyne is achieved by adding a bromoalkane into a mixture of sodium amide and an alkyne. The bromoalkane and alkyne respectively are