Which of the following reagents,when heated with ethyl chloride,forms ethylene?

The major product of the following reaction is:

Which organic chloro compound shows complete stereochemical inversion during an $S_N2$ reaction?

Enlist the main points of difference between $S_{N}1$ and $S_{N}2$ reactions.

Predict the order of reactivity of the following compounds in $S_N1$ reaction :
$(I)$ $CH_3-CH_2-CH(Br)-CH_3$
$(II)$ $(CH_3)_2CH-CH_2-Br$
$(III)$ $(CH_3)_3CBr$

Identify the major product $(P)$ in the following reaction sequence.
$(CH_3)_3 CBr$ $\xrightarrow{\text{Alcoholic } KOH, \Delta}$ $\xrightarrow{HBr} P$