Which of the following hydrocarbons will be in a liquid state at room temperature?

The compound with molecular formula $C_{20}H_{42}$ is

The number of different substitution products possible when bromine and ethane are allowed to react is:

$Ph-CH_2-CH(D)-CH_3 \xrightarrow{Br_2/hv}$
Product of the above reaction will be:

Arrange the following alkanes in decreasing order of their heats of combustion:
$(i)$ $CH_3-C(CH_3)_2-CH_3$ (Neopentane)
$(ii)$ $CH_3-CH(CH_3)-CH_2-CH_3$ (Isopentane)
$(iii)$ $CH_3-CH_2-CH_2-CH_2-CH_3$ ($n$-pentane)

$A$ sample of $2,3-$dibromo$-3-$methylpentane is heated with zinc dust. The resulting product is isolated and then heated with $HI$ in the presence of red phosphorus. Indicate the structure that represents the final organic product formed in the reaction.