This common seki shape has some interesting properties, both in Chinese rules and in Japanese rules.
Under Japanese rules, it’s a mistake for black to play B - white gains a point by capturing that stone. Black should therefore just leave the situation as it is. White can choose to also leave it, or to play at A and initiate a trade, which would not affect the score: white gains 4 points in the corner (2 territory + 2 captures), while black gains 4 points around B (2 territory + 2 captures).
Under Chinese rules, it would be a mistake for white to play A. In the trade, black’s area becomes 1 point bigger (from 3 points in the corner to 4 points around B), whereas white’s total area stays the same. Therefore, white should just leave the situation as it. Black can choose to also leave it, or to play at B, which is forcing and does not lose any points.
Keeping all that in mind, consider this position:
Black to play, no komi, what’s the result?
Does the answer depend on the ruleset?