In the deadlines it says challenge b only because, challenge A isn’t marked on speed, well, sort of, you have to do challenge A before the challenge closes to be at the top of the challenge A leader board (challenge A doesn’t count towards much, you don’t even have to do it, but it helps you in challenge B and if you do it quickly, you can solve challenge B quite quickly), challenge B is marked on speed, so the quicker you do challenge B, the higher you will be on the leader board.
I hope that all makes sense to you and I hope that answers your question of “why in the deadlines it says b challenge only?”.
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