The major product of the following reaction is:

Product $(A)$ is :

For the reaction $CH_3Br + Nu^{-} \rightarrow CH_3 - Nu + Br^{-}$,arrange the following nucleophiles $(Nu^{-})$ in the decreasing order of their reaction rate: $(1) PhO^{-}, (2) AcO^{-}, (3) HO^{-}, (4) CH_3O^{-}$.

Which alkyl halide undergoes the ${S}_{N}1$ reaction most rapidly?

$2$-chlorobutane $+ Cl_2 \rightarrow C_4H_8Cl_2$ (isomers)
Total number of optically active isomers shown by $C_4H_8Cl_2$,obtained in the above reaction is......................

The major product obtained from $E_{2}$-elimination of $3$-bromo-$2$-fluoropentane is