Answer:
RuBP made during the dark reaction is used to start the reaction again.
Explanation:
Dark reactions start with carboxylation of RuBP (ribulose 1,5 bisphosphate) in presence of enzyme RuBisCo. The regeneration phase of dark reactions forms the RuBP again which then enter the next round of dark reactions. Since the initial substance (RuBP) is reformed during dark reactions, they form a cyclic pathway.