In the experimental design, there are multiple reactions coupled together. which reaction is the primary reaction of interest for the overall experiment?

a. starch + l₃⁻ → starch-l₃⁻complex
b. 2Fe³⁺ + 3l → 2Fe²⁺ + l₃⁻
c. l₃ + 2S₂O₃²⁻ → 2l² + S₄O₆²⁻
d. 2Fe³⁺ + 3l + 2S₂O₃²⁻ → l₃ + S₄O₆²⁻ + 2Fe²⁺