Photosynthesis question, IB 2016

Which molecule would first contain 14C if the alga Chlorella was grown in the presence of light and radioactive CO2?

A. Glycerate 3-phosphate
B. Glucose
C. Rubisco
D. Ribulose bisphosphate (RuBP)

For this question I immediately crossed out B and C but I could not figure which one was correct between A and D. I chose D but correct answer is A. Can someone explain why? Thanks

RuBP is converted to Glycerate 3-phosphate by the enzyme RuBisCO under fixation/incorporation of CO2. This process happens in the Calvin Cycle. Therefore A is the answer.