The molecule $2-$bromo$-3-$chlorobutane has two stereocenters $(C2$ and $C3)$ and thus can exist as a maximum of four different stereoisomers. The Fischer projections of these stereoisomers are shown below. Answer the questions which follow.

$"A"$

$"$B$"$

$"$C$"$

$"$D$"$

a$.$ Which stereoisomers $($if any$)$ represent pairs of enantiomers? $($list all the pairings that apply$)$

b$.$ Which stereoisomers $($if any$)$ represent pairs of diastereomers? $($list all the pairings that apply$)$

c$.$ Which stereoisomers $($if any$)$ represent a meso compound?