question 91 from destroyer

[issa]

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choice b confuses me. why did he make 2 switches instead of 1 switch?

in the first switch the hydrogen was in the back so i am confused on why he did a 2nd switch.

 

[bxs023100]

bc if you only do one switch, the molecules are diasteromers. in order for them to remain the same molecule/enantiomers, you have to do an even number of switches