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$W,X$ 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?

- $S \cdots \cdots \overset{Z1}{\rightarrow}W\overset{Z2}{\rightarrow}X\overset{Z3}{\rightarrow} Y\overset{Z4}{\rightarrow}Z$
- $S \cdots \cdots \overset{Z4}{\rightarrow}W\overset{Z2}{\rightarrow}X\overset{Z1}{\rightarrow} Y\overset{Z3}{\rightarrow}Z$
- $S \cdots \cdots \overset{Z3}{\rightarrow}W\overset{Z1}{\rightarrow}X\overset{Z2}{\rightarrow} Y\overset{Z4}{\rightarrow}Z$
- $S \cdots \cdots \overset{Z4}{\rightarrow}W\overset{Z1}{\rightarrow}X\overset{Z2}{\rightarrow} Y\overset{Z3}{\rightarrow}Z$