Which of the following conditions are favour for ${\mathrm{S}}_{\mathrm{N}}2$ mechanism in alkyl halides?

Identify correct steps representing ${\mathrm{S}}_{\mathrm{N}}1$ mechanism for the cleavage of ether with $\mathrm{HI}$.

The relative rate of acid catalysed dehydration of following alcohols would be:

(I) 

(II) 

(III) 

(IV)