caranthir:
The choices in a real game would be to make a ko with A19 or play elsewhere. Us playing C18 or C19 would be equivalent to a pass, since the opponent can't live with points anyway (unless we started the ko and lost it). The opponent can play C19 or C18 if they want to absolutely prevent the possibility of ko. Usually they wouldn't play C19 or C18 at least before the very end, because the group is extremely hard to kill with ko. Almost always the situation is just left as is and at the end of the game it's deemed seki anyway. For explanation, you can look at "Ten thousand year ko" at Sensei's Library, for instance.