Hydride of boron occurs as B_2 H_6 but B_2 Cl_6 does not exist....

Options

1. A$p \pi-d \pi$ back bonding is possible in $\mathrm{B}_2 \mathrm{H}_6$ but not in $\mathrm{B}_2 \mathrm{Cl}_6$
2. Bboron and hydrogen have almost equal values of electronegativity
3. Cboron and chlorine have almost equal atomic sizes
4. Dsmall hydrogen atoms can easily fit in between boron atoms but large chlorine atoms do not.