W, X and Y are the intermediates in a biochemical pathway as shown below:
$$S \cdot \cdot \cdot \cdot \cdot \cdot \cdot \cdot \to W \to X \to Y \to Z$$
Mutants auxotrophic for Z are found in four different complementation groups, namely Z1, Z2, Z3 and Z4. The growth of these mutants on media supplemented with W, X, Y or Z is shown below (Yes: growth observed: No: growth not observed):

Mutants	Media Supplemented with Mutants
	W	X	Y	Z
Z1	No	No	Yes	Yes
Z2	No	Yes	Yes	Yes
Z3	No	No	No	Yes
Z4	Yes	Yes	Yes	Yes

What is the order of the four complementation groups in terms of the step they block?