Question

3. ―Since you didn't agree with him,why didn't you raise objections at the meeting?

―Well,now I regret        (do) that.

Answer 1

3. not doing句意："既然你不同意他的意见，在会上为什么不提出反对呢？""我现在后悔没那样做了。"regret do?ing sth"后悔做过某事"，根据句意可知要用否定形式。