Isomers have the same ...........

Which of the following pairs are not diastereomers?

The structure of $E-(S)-1$-chloro-$1$-bromo-$2$-methyl-$2$-hexene is best represented by

Identify the name of the method used for the three-dimensional representation of the molecule as follows:

How many different stereoisomers are possible for the following compound?
$Cl-CH=CH-CH(Cl)-CH=CH-Cl$

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 $(Y)$.