Why Does Rubisco Show Relatively Little Preference for CO₂ as a Substrate

Multiple Choice

Why does Rubisco show relatively little preference for CO₂ as a substrate over O₂?

A) Both CO₂ and O₂ bind directly to Rubisco's active site.
B) CO₂ and O₂ bind to RuBP,which occupies the active site; their ability to attack RuBP is roughly equal.
C) Rubisco binds to a regulatory site.
D) CO₂ and O₂ bind to a regulatory site instead of the active site.
E) Both O₂ and CO₂ are repelled RuBP.