When `O_(2)` is adsorbed on a metallic surface, electron transfer occurs form the metal to `O_(2).` The true statement (s) regarding this adsorption is (are)
A. `O_2` is physisorbed
B. heat is released
C. Occupency of `pi _(2p)` of `O_2` is increased
D. bond length of ` O_2` is increased