No.708(Answers)

Black 1 is crucial to get a Ko.

Black 1 is wrong.
With White 2 and 4, Black dies.

Black 1 is the vital point.
If White2, Black 3 and 5 are crucial.
White has to create a Ko with 8.

Black 5 allows White to live with 6 to 10, utilizing the White stone in the corner.