Acetylene is prepared industrially by passing electric discharge through graphite electrodes in the atmosphere of

$C_2H_2 \mathop {}$ ${\xrightarrow{{HgSO_4, H_2SO_4}}} A$ $\xrightarrow{{[O]}} B$. $B$ is

$2$-Hexyne reacts with $.......$ to give $trans-2$-hexene.

The $-C \equiv C-$ bond is found in

One mole of $1,2$-dibromopropane on treatment with $X$ moles of $NaNH_2$ followed by treatment with ethyl bromide gave a pentyne. The value of $X$ is

The product$(s)$ obtained via oxymercuration $(HgSO_4 + H_2SO_4)$ of $1$-butyne would be: