Which of the following gases is used for welding?

$C_2H_2$ $\xrightarrow[1\% HgSO_4]{dil. H_2SO_4} A$ $\xrightarrow{[O]} B$ $\xrightarrow{NaOH} C$ $\xrightarrow{NaOH/CaO} D$,$[D]$ is:

The major product $(A)$ is:

When iodoform is heated with $Ag$ powder,the product formed is:

In the following reaction,$Z$ is known as: $CH \equiv CH \xrightarrow{Z} CH_3CHO$

$CH_3-C \equiv C-CH_3 \xrightarrow{(i) X, (ii) H_2O/Zn} CH_3-CO-CO-CH_3$. $X$ in the above reaction is: