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 $\pi_{2 p }^*$ of $O _2$ is increased

$(D)$ bond length of $O _2$ is increased

• A

  $(B,C,D)$

• B

  $(A,B,C)$

• C

  $(A,B,D)$

• D

  $(A,C,D)$