$A$ hydrocarbon $X$ with molecular formula $C_4H_6$ decolourises bromine water and forms a white precipitate in ethanolic $AgNO_3$ solution. Treatment of $X$ with $HgCl_2$ in aqueous $H_2SO_4$ produces a compound,which gives a yellow precipitate when treated with $I_2$ and $NaOH$. The structure of $X$ is

• A
  $CH_2=CH-CH=CH_2$
• B
  $CH_2=C=CH-CH_3$
• C
  $CH_3-C \equiv C-CH_3$
• D
  $CH_3-CH_2-C \equiv CH$