${\mathrm{BCl}}_3$ does not exist  as dimer but ${\mathrm{BH}}_3$ exist as dimer $\left({\mathrm{B}}_2{\mathrm{H}}_6\right)$ because

• A

  Chlorine is more electronegative than hydrogen

• B

  There is $\mathrm{p\pi }-\mathrm{p\pi }$ back bonding in ${\mathrm{BCl}}_3$ but ${\mathrm{BH}}_3$ does not contain such multiple bonding

• C

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

• D

  None of the above