Dehydrohalogenation in monohaloalkanes produces

Which of the following statements is true in the case of alkyl halides?

Which among the following alkyl halides gives more than one product upon dehydrohalogenation?

The reactivity of $2-$bromo$-2-$methylbutane $(I)$,$1-$bromopentane $(II)$ and $2-$bromopentane $(III)$ towards $S_N2$ displacement is such that :

$A$ bromoalkane $X$ reacts with magnesium in dry ether to form compound $Y$. The reaction of $Y$ with methanal followed by hydrolysis yields an alcohol having molecular formula $C_{4}H_{10}O$. The compound $X$ is

In the following pairs of halogen compounds,which compound undergoes faster $S_N 1$ reaction?