The number of stereoisomers possible for the following compound is $CH_3-CH=CH-CH(Br)-CH_2-CH_3$

The correct statements with respect to the above pair of reactions are that:
$(I)$ The reactions are stereospecific.
$(II)$ $(X)$ is erythro and $(Y)$ is threo isomer.
$(III)$ $(X)$ is threo and $(Y)$ is erythro isomer.
$(IV)$ Each of $(P)$ and $(Q)$ gives a mixture of $(X)$ and $(Y)$.

What is the decreasing order of the inductive effect for the following benzyl/allyl systems (where $R$ is an alkyl group)?

Which of the following pairs of compounds are enantiomers?

Which types of isomers of $C_4H_8$ are possible? Give their names and structures.

Determine the correct priority order for the following compound to assign the $R$ or $S$ configuration.
$\begin{array}{c} CHO \\ | \\ H-C-OH \\ | \\ CH_2OH \end{array}$