I still interpreted (B) as in a 12 month period since that is how 5->4 works, but that is an interesting point and I can see how it could be interpreted that way.
That is not how 5->4 works. It's 10 races over any time period. This is how the Cat 4->3 upgrade on experience works. 25 races (including your 10 Cat 5 races) plus 10 top-10 finishes.