All alkyl halides undergo reduction process except

Which of the following alkyl halides undergoes elimination reaction readily?

Which of the following alkyl halides mainly undergo reaction by SN^2 mechanism.

Which of the following alkyl halides undergoes S_N1 reaction the fastest?

Some alkyl halides undergo substitution whereas some undergo eleimination reaction on treatment with bases. Discuss the structural freature of alkyl halides with the help of examples which are responsible for the difference.