The following reaction is an example of a ....... reaction: $CH_2Br-CH_2Br + 2KOH (alc.) \xrightarrow{\Delta} CH \equiv CH + 2KBr + 2H_2O$

Propyne reacts with excess $HBr$ to form the compound $Z$. The structure of $Z$ is:

When acetylene reacts with $HCl$ in the presence of $HgCl_2$,the product formed is.........

The relation between $(B)$ and $(C)$ is:
$CH_3-C \equiv CH$ $\xrightarrow{NaNH_2}$ $\xrightarrow{CH_3-I} (A)$ $\xrightarrow{Li/liq. NH_3} (B)$
$CH_3-C \equiv CH$ $\xrightarrow{NaNH_2}$ $\xrightarrow{CH_3-I} (A)$ $\xrightarrow[Pd/CaCO_3]{H_2} (C)$

Provide the chemical reactions for the following conversions:
$(1)$ Propyne from ethyne
$(2)$ $2-$butyne and $1-$butyne from ethyne

The reagent which is used to distinguish between propene and propyne is