shevious: Black , combination is critical.
White is in the shortage of liberties.
Black can be played at .
Other resistances of White can be a or b, which lead to the same conclusion.
hhw: For a, if by the same conclusion you mean that White dies, then yes. But it obviously does not revert to the main line above, nor does it end in White being short of liberties (at least I don't see that).
Black first is not good.