$W,X$ and $Y$ are the intermediates in a biochemical pathway as shown below:

$$S \cdots \cdots \rightarrow W\rightarrow X\rightarrow Y\rightarrow Z$$

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 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?

1. $S \cdots \cdots \overset{Z1}{\rightarrow}W\overset{Z2}{\rightarrow}X\overset{Z3}{\rightarrow} Y\overset{Z4}{\rightarrow}Z$
2. $S \cdots \cdots \overset{Z4}{\rightarrow}W\overset{Z2}{\rightarrow}X\overset{Z1}{\rightarrow} Y\overset{Z3}{\rightarrow}Z$
3. $S \cdots \cdots \overset{Z3}{\rightarrow}W\overset{Z1}{\rightarrow}X\overset{Z2}{\rightarrow} Y\overset{Z4}{\rightarrow}Z$
4. $S \cdots \cdots \overset{Z4}{\rightarrow}W\overset{Z1}{\rightarrow}X\overset{Z2}{\rightarrow} Y\overset{Z3}{\rightarrow}Z$