${\mathrm{BCl}}_3\text{ does not exist as a dimer but }{\mathrm{BH}}_3\text{ exist as, }{\mathrm{B}}_2{\mathrm{H}}_6\text{ because }$

• A

  Chloride is more reactive than Hydrogen

• B

  Large size chlorine atoms does not fit between small sixed boron atoms

• C

  $\text{ There is p}\pi -p\pi \text{ back bonding in }{\mathrm{BCl}}_3$

• D

  Both 2 and 3