Which alkyl halide from the following pairs would you expect to react more rapidly by an `S_N2` mechanism ? Explain your answer.
(i) `underset((A))(CH_3CH_2Br) " or " underset((B))underset(Br)underset(|)(CH_3CH_2CHCH_3)`
(ii) `underset((C))underset(Br)underset(|)(CH_3CH_2CHCH_3) " or " underset((D))underset(CH_3)underset(|)(H_3C-C-Br)`
(iii) `underset((E))underset(CH_(3))underset(|)(CH_3CHCH_2CH_2Br) " or " underset((F))underset(CH_3)underset(|)(CH_3CH_2CHCH_2Br)`

Explanation

(i) A, (ii) C and (iii) E