Black to capture "a" or "b"

These are the intersections MadLab considers while searching:

The answer is Black 1 (see the figure below), which MadLab finds in about a second on a fast pc. If White answers at 2, Black has the tesuji at 3. This does not capture the white b-block unconditionally, but if White choses to rescue it, the other block (a) dies in the process. In this problem, remember that the five black stones to the left of b are vulnerable. In fact, the problem could be paraphrased into "how to save these five black stones".

Source: This problem is extracted from a game between Rui Naiwei (B) and Jiang Shujiu (Jujo) (W).