Which gas is used in metal welding?

$CH_{3}-C \equiv CH$ $\xrightarrow{2 HBr}$ $\xrightarrow{H_{2}O} \text{Product}$. The product is:

$KMnO_4$ will oxidise acetylene to

Give the main product of the following reactions:
$(i)$ $1,1,2,2-$tetrabromopropane + zinc powder in methanol.
$(ii)$ Reaction occurring on passing hydrogen gas through carbon electrodes at high temperature in an electric arc.

What will be the product of the reaction $CH_3-C \equiv C-CH_2CH_3 \xrightarrow[(2) \text{ Hydrolysis}]{(1) O_3} \, ?$

In which of the following,the bond length between the hybridized carbon atom and the other carbon atom is minimum?