Dehydrohalogenation of an alkyl halide is a/an

The best method for the preparation of $Me_{3}CCN$ is

The decreasing order of the rate of $S_{N^1}$ reaction for the following compounds is:
$I$. $Ph-C(Br)(CH_3)-Ph$
$II$. $Ph-CH(Br)-Ph$
$III$. $Ph-CH(Br)-CH_3$
$IV$. $Ph-CH_2Br$

$A$ 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

On treatment with chlorine in the presence of sunlight,toluene gives the product

$C_6H_6 + CH_3CH_2CH_2Cl + AlCl_3 \to X$ (Major product)
$X$ (Major product) $\xrightarrow{HNO_3 + H_2SO_4} Y$ (Major product) $\xrightarrow{Sn + HCl} Z$ $\xrightarrow{H_2SO_4 + SO_3} W$
$W$ is