Whatever action this person C takes in respect of Groups A and B, his eventual attainment of magga phala will prove beneficial not only to himself, but to other beings eventually. The collective future suffering he saves others from outweighs immeasurably any suffering they may undergo now.
Should he help just one person get onto the Path, the suffering prevented in this way in the apayas will be many times over what all those people suffer now at the hands of Group B. Their suffering is after all only in the human realm.
So, for me it’s: “Strive/attain magga phala for oneself while helping others do the same”
Thank you Axel.
Much merits to you.