Why Does Rubisco Make Up About Half of the Protein

Why does rubisco make up about half of the protein content in a typical chloroplast?

A) most of the enzyme is in an oxidized state, waiting for the production of NADPH to reactivate it
B) it has an extremely high K_m for CO₂, thus a large number of enzymes increased the chance of substrate binding
C) since oxygen is also a substrate, those enzymes that bind oxygen are sequestered
D) it is a very inefficient enzyme, catalyzing only three reactions per second
E) none of the above