Which of the following decolorizes $Br_2$ water solution?

Ethyne can be obtained from.......

In the reaction $CH_3-CH_2-C \equiv CH + HCl \to B \xrightarrow{HI} C$,what is the product $C$?

An organic compound $X$ with molecular formula $C_6H_{10}$,when treated with $HBr$,forms a $gem$-dibromide. The compound $X$ upon warming with $HgSO_4$ and dil. $H_2SO_4$,produces a ketone,which gives a positive iodoform test. The compound $X$ is

Propyne on cyclic polymerisation gives

An unsaturated hydrocarbon $X$ on ozonolysis gives $A$. Compound $A$ when warmed with ammoniacal silver nitrate forms a bright silver mirror along the sides of the test tube. The unsaturated hydrocarbon $X$ is ..... .