In the Mechanism of Rubisco, What Role Does the Mg²⁺ $\beta$

In the mechanism of Rubisco, what role does the Mg²⁺ play?

A) neutralizes negative charges of active site Asp and Glu, enabling ionization of CO₂
B) activates water molecule that hydrates the carboxy- $\beta$ -keto intermediate
C) complexes with the phosphate on C-1 to hold ribulose-1,5-bisphosphate in the active site
D) repels the product, 3-phosphoglycerate, allowing it to leave the active site more rapidly
E) none of the above