When $$O_2$$ is adsorbed on a metallic surface, electron transfer occurs from the metal to $$O_2$$. The TRUE statement(s) regarding this adsorption is(are)

A

$$O_2$$ is physisorbed

B

heat is released

C

occupancy of $$\pi_{2p}^*$$ of $$O_2$$ is increased

D

bond length of $$O_2$$ is increased