stage前的介詞是in，at

前面的介詞是at，例如：

1、We're now in the process of developing the function further， so that at stage three we'll be able to issue invoices.我們現在正在進一步完善該功能，以便在第三階段能夠解決發票問題。

2、People tend to work hard at this stage of life.人在這個人生階段往往發奮努力。

3、At one stage it looked as though they would win.有一段時間，他們好像大有獲勝的希望。