The colour of the $$X_2$$ molecules of group 17 elements changes gradually from yellow to violet down the group. This is due to

A

the physical state of $$X_2$$ at room temperature changes from gas to solid down the group

B

decrease in ionization energy down the group

C

decrease in $$\pi^* - \sigma^*$$ gap down the group

D

decrease in HOMO-LUMO gap down the group