I have always used B,for no particular reason.because of reasons.
The Mode VI Kyrie, B or C depending on the book you're using, is very congregational-friendly.
Interestingly here is the Mass where we first introduced the Mode VI Kyrie from Mass XVII. You can hear the ease with which the congregation picks it up.
https://www.facebook.com/cathedralchoirphoenix/videos/1675187846044774/?permPage=1
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