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

The number of $\pi$-bonds in the product formed by passing acetylene through dilute sulphuric acid containing mercuric sulphate is

$A$ metallic carbide on treatment with water gives a colourless gas which burns readily in air and which gives a precipitate with ammoniacal silver nitrate solution. The gas evolved is:

$A$ bromoalkane is reacted with the sodium salt of an alkyne to obtain $3$-octyne. What are the bromoalkane and the alkyne respectively?

What is the increasing order of acidity for the following substances?
$(1)$ $CH_2=CH_2$
$(2)$ $CH_3-CH_3$
$(3)$ $HC \equiv CH$
$(4)$ $NH_3$

Which of these will not react with acetylene?