理论上讲都可以,但是I've just finished my lunch更符合英美表达习惯
现在完成时I have just finished my lunch
I have just had lunch.我觉得现在完成式比较好。刚吃过，所以不饿呢！
都可以，看别人怎么问？一般问， have you.....你就回答 I 've ....
