In the given reaction: $CH_3-CH(OH)-C(OH)(CH_3)_2 \xrightarrow{HIO_4} (a) + (b)$. $(a)$ and $(b)$ respectively are:

$A$ Grignard reagent $(X)$ on reaction with a carbonyl compound $(Y)$ followed by hydrolysis gives $Z$. $Z$ reacts with conc. $HCl$ at room temperature to give immediate turbidity. $X$ and $Y$ respectively are:

The alcohol that produces turbidity immediately with $ZnCl_2 +$ conc. $HCl$ at room temperature is:

The number of possible alcoholic isomers for $C_4H_{10}O$ are

The best method for the conversion of an alcohol into an alkyl chloride is by treating the alcohol with

Consider the following alcohols:
$(I)$ $C_6H_5CH_2OH$
$(II)$ $O_2N-C_6H_4-CH_2OH$ (para-nitro)
$(III)$ $CH_3O-C_6H_4-CH_2OH$ (para-methoxy)
$(IV)$ $Br-C_6H_4-CH_2OH$ (para-bromo)
The order of decreasing reactivities of these alcohols towards nucleophilic substitution with $HBr$ is: