…? — that's rightA conversational pattern: の? makes the first speaker’s question sound explanatory or confirmatory, and そう begins an affirmative reply. The reply may be expanded, as in そうだよ or そうでもない.
JA: もう宿題をしたの?」「そう、さっき終わったよ。
EN: Have you done your homework already? — That's right; I finished a little while ago.