Reactant \( A \) converts to product \( D \) through the given mechanism (with the net evolution of heat):
\( A \rightarrow B \) slow; \( \Delta H = +\text{ve} \)
\( B \rightarrow C \) fast; \( \Delta H = -\text{ve} \)
\( C \rightarrow D \) fast; \( \Delta H = -\text{ve} \)
Which of the following represents the above reaction mechanism?