Which of the following is $NOT$ obtained when a mixture of bromomethane and bromoethane is treated with sodium in dry ether?

$(i)$ Give the reaction for the bromination of ethane.
$(ii)$ Is this reaction suitable for obtaining pure bromoethane? Why?
$(iii)$ Give the name of the reaction mechanism and the names of the stages occurring in the reaction.
$(iv)$ Give the reaction for the first stage.
$(v)$ Which type of bond cleavage occurs in the first stage?

How many primary,secondary and tertiary carbon atoms respectively are present in isobutane?

Which one of the following compounds does not give addition reactions?

$A$ fuel has the same knocking property as a mixture of $70\%$ isooctane and $30\%$ $n$-heptane. The octane number of the fuel is .............

$C-C$ bond length is maximum in