BCl₃ does not exist as dimer but BH₃ exist as dimer (B₂H₆) because:

chlorine is more electronegative than hydrogen

there is p$\mathrm{\pi }$- p$\mathrm{\pi }$ back bonding in BCl₃ but BH₃ does not contain such multiple bonding

large sized chlorine atoms do not fit in between the small boron atoms whereas small sized hydrogen atoms get fitted between boron atoms

none of the above