Among $CH_3F$,$CH_3Cl$,$CH_3Br$,and $CH_3I$,which one does not give methane on reduction?

Increasing the concentration of a nucleophile in a typical $S_N2$ reaction by a factor of $10$ will cause the reaction rate to

Arrange the following compounds in decreasing order of reactivity for nucleophilic substitution reaction:
$(I)$ $CH_3-CH_2-OSO_2CF_3$
$(II)$ $CH_3-CH_2-OTs$
$(III)$ $CH_3-CH(OH)-CH_3$
$(IV)$ $C_6H_5-CH(OH)-CH_3$

The rate of $S_{N}1$ reaction for the given compounds is:

Dehydrohalogenation in the presence of $OH^{-}$ is correctly represented by:

Which of the following is most reactive towards $S_{N}1$ reaction?