I thought there were 3 kyrie in mass xvii. I recall singing option C. The different settings of the credo wre ad libitum as far as I know. I prefer credo i.
In more modern books three Kyrie can be found for Mass XVII, and two for Mass XI... etc.
If you look you will see that a couple are very similar and the dating of them implies that there is an older and newer version.
In the Older books the three / two different Kyrie are still included, but the older versions are found in the Ad lib. section, not with the rest of the Mass.
Anyway we sing Credo II during Lent / Advent, as it sounds less joyful, than the others.
In the 1908 Graduale Romanum, the following note was inserted after the four versions of the Credo:
"This Ordinary is NOT meant to be a matter of hard and fast rule: chants from one Mass may be used together with those from others, the Ferial Masses excepted. In the same way, in order to add greater solemnity, one or more of the following “Chants ad libitum” may be employed." (translation from Liber Usualis.)
I'm not recommending singing something besides Mass 17, but people should know that there isn't a rule requiring it. Also, sacred polyphony is always allowed, regardless of season.
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