Melvin Calvin Received a Noble Prize for Unravelling the Reaction

Melvin Calvin received a Noble Prize for unravelling the reaction sequence of the production of sugar in plants. How was he able to do this?

A) using tritium to slow the reactions down
B) taking multiple x-rays as a plant is growing
C) using a radioactive tracer to follow the carbon throughout the plant
D) growing a plant by only using human breath
E) using Carbon-14 dating