Rubisco catalyzes the reaction that fixes a carbon dioxide molecule into a ribulose 1,5-bisphosophate backbone, which immediately splits into two 3-carbon intermediates. As the cycle progresses, another 3-carbon product is formed, G3P. Only one out of six G3Ps will be used to make glucose or other biosynthetic products. Why? Choose one: A. The G3P serves as a catabolic substrate. B. Making glucose takes all six G3P. C. The G3P immediately splits into carbon dioxide and a 2-carbon product. D. The ribulose 1,5-bisphosphate must be regenerated.