When $2$-butyne is treated with $Pd-BaSO_4$ (Lindlar's catalyst),the product formed will be:

Consider the following sequence,where $A$ can be:
$A$ $(C_{3}H_{6}Cl_{2})$ $\xrightarrow[2. \, H^{+} \, (mild)]{1. \, \text{alcoholic } KOH / \Delta, \text{ followed by } NaNH_{2} / \Delta}$ $B$ $\xrightarrow[H_{2}O_{2} / OH^{-}]{B_{2}H_{6}}$ $C$
$A$ $\xrightarrow{aq. \, KOH \, (excess)}$ $C$
Identify $A$.

Silver acetylide when heated with $HCl$ gives:

$Ph-C \equiv CH \xrightarrow[MeOH]{MeO^{-}}$ The major product of the reaction is:

$CH \equiv CH + HBr \to X$,the product $X$ is:

Acetylene molecules contain