Calvin and colleagues determined the pathway of carbohydrate synthesis in plants by studying the incorporation of radioactive carbon dioxide into biological compounds. Suppose that photosynthesis is proceeding at a steady pace in a typical experiment with the lights on, and carbon dioxide is being combined with ribulose-bisphosphate (RuBP) to produce 3-phosphoglycerate (3PG). Then suddenly the source of carbon dioxide is eliminated. What changes in the concentrations of 3PG and RuBP would occur?

A. 3PG levels rise, RuBP levels fall.

B. 3PG levels fall, RuBP levels rise.

C. 3PG levels rise, RuBP levels rise.

D. 3PG levels fall, RuBP levels fall.

E. 3PG levels rise, RuBP levels stay the same.