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