According to MO theory, which of the following lists ranks the oxygen species in terms of decreasing bond order?

a) O₂ > O₂⁺ > O₂⁻
b) O₂⁻ > O₂⁺ > O₂
c) O₂⁺ > O₂ > O₂⁻
d) O₂⁻ > O₂ > O₂⁺