How many bond cleavages are required to convert cubane into a non-cyclic skeleton?

Relationship between products $(Q)$ and $(S)$ is
$CH_3-CH=CH_2$ $\xrightarrow[H_2O_2, OH^{-}]{BH_3, THF}$ $(P)$ $\xrightarrow[CH_2Cl_2]{PCC}$ $(Q)$
$CH_3-CH=CH_2$ $\xrightarrow[NaBH_4, HO^{-}]{Hg(OAc)_2, H_2O}$ $(R)$ $\xrightarrow[CH_2Cl_2]{PCC}$ $(S)$

What is the decreasing order of acidic strength for ethane $(I)$,ethene $(II)$,ethyne $(III)$,and propyne $(IV)$?

Provide the classification of hydrocarbons.

Arrange the following in the correct order of their acidic strength:

$I. H_2C=CH_2$

$II. CH \equiv CH$

$III. CH_3-C \equiv CH$

$IV. CH_3-CH_3$

The correct order of heats of combustion of the following compounds is:
$(I)$ Cyclopropane
$(II)$ Cyclobutane
$(III)$ Cyclopentane