If the bond dissociation energies of $XY, X_2$ and $Y_2$ (all diatomic molecules) are in the ratio of $1 : 1 : 0.5$ and $\Delta H_f$ for the formation of $XY$ is $-200 \; kJ \; mol^{-1}$. The bond dissociation energy of $X_2$ will be :

Question

( A ) $400 \; kJ\; mol^{-1}$
( B ) $200 \; kJ\; mol^{-1}$
( C ) none of these
( D ) $300 \; kJ\; mol^{-1}$