Which of the following best defines a "common intermediate" in coupled chemical reactions?
a) A catalyst that lowers the activation energy of both reactions but is regenerated unchanged
b) A high‐energy transition state that both reactions pass through
c) A transient species produced in one reaction step and consumed in a later step, so it cancels from the overall equation
d) A spectator ion that appears on both sides of the coupled equations and is omitted from the net reaction