The Mechanism for the Reaction

The mechanism for the reaction

in the presence of I^-(aq) is proposed to be:

Step 1: (slow)
Step 2: (fast)
What is the molecularity of the rate-determining step?

A) zero molecular
B) unimolecular
C) bimolecular
D) termolecular
E) More information is needed to answer this question.