When primary alcohol is oxidized with chlorine,it produces

$CH_3CH_2OH$ $\xrightarrow{Red \ P + I_2} (A)$ $\xrightarrow[Ether]{Mg} (B)$ $\xrightarrow{HCHO} (C)$ $\xrightarrow{H_2O} (D)$
The compound $(D)$ is

$1, 2, 3$-butanetriol undergoes oxidative cleavage with $HIO_4$. During this process:

Primary,secondary and tertiary alcohols can be distinguished by$-$

An alcohol $X$ $(C_5H_{12}O)$ on dehydration gives $Y$ (major product). Reaction of $Y$ with $HBr$ gave $Z$ ($C_5H_{11}Br$,major product). $Z$ undergoes nucleophilic substitution in two steps. What are $X$ and $Y$?

Which of the following compounds gives a positive iodoform test with $I_2$ and $NaOH$?