Ivan Detkov: A2 is not an approach ko. It followed by connection and E1, which is a direct ko. However, E1 is worse than A2. The difference is who takes ko first. I corrected the solution.
guile: Ko sure are tricky... Why W would not connect at A1 instead, as B has no external lib ? I thought it was a choice because i did not think B is forced to play this ko, but can refresh it for the bent4 Ko ? That's why i did not see any difference (B takes first in both cases).
promme: After C19A19, if black plays A18 it is approach ko but plays E19 is direct ko, right ?
So E19 should not be a correct answer
guile: I think your comment is very interesting. It seems to me it is rather a choice, not a (Good direct VS Bad approach) Ko option. The Ko is in fact the same for both player depending on the attacher choice.
I will need a stonger player to confirm or infirm that.
promme:
Thanks Ivan and guile!