This game is from the Chinese professional tournament: 1st Jincheng Professional Weiqi Open Tournament, round 4, Wang Shiyi 8p played as black versu Dang Yifei 9p. It is black's turn, how could black live the big group?
Playing at A is a normal sense, as it is a tiger mouth with many eye shapes, but black could at most achieve a ko afterwards, check out the variations with the hints.
Playing at B is the unexpected good move, E18 and H16 become a miai, so black could make an eye to live. This question should not be too hard for high dan players, but as a professional game, black should have foresee this variation long time ago, so his group could live successfully.