Ethyne on reaction with dil. $H_2SO_4$ and $Hg(II)$ gives:

Hydrogenation of the compound shown below in the presence of a poisoned palladium catalyst (Lindlar's catalyst) gives:

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

Oxidation of propyne with $SeO_2$ gives ....

An unknown compound $(A)$ has a molecular formula $C_4H_6$. When $(A)$ is treated with excess of $Br_2$, a new substance $(B)$ with formula $C_4H_6Br_4$ is formed. $(A)$ forms a white precipitate with ammoniacal silver nitrate solution. $(A)$ may be:

In which of the following molecules,$2$ $\pi$ bonds are present?