Consider the Three-Step Mechanism for the Reaction of a Through

Consider the three-step mechanism for the reaction of A through intermediates B and C to produce D shown below.
A → B E_a = 15 kcal/mol, ΔH° = 13 kcal/mol
B → C E_a = 10 kcal/mol, ΔH° = -6 kcal/mol
C → D E_a = 2 kcal/mol, ΔH° = -20 kcal/mol
Which of the three steps is rate-limiting?

A) The reaction of A to B.
B) The reaction of B to C.
C) The reaction of C to D.
D) All three steps occur at the same rate; there is no rate-limiting step.
E) The most exothermic step is rate-limiting.