An overall reaction follows the four-step mechanism shown below:
Step 1 P + Q → R
Step 2 R + S → T + U
Step 3 U → V + P
Step 4 V + W → X
Which statement correctly classifies the roles of species P, R, and U in the mechanism?
a) P is a reactant; R is a catalyst; U is a reaction intermediate.
b) P is a catalyst; R and U are reaction intermediates.
c) P and R are intermediates; U is a product of the overall reaction.
d) All three species P, R, and U are intermediates; none acts as a catalyst.