Will you be back ----five in the afternoon?A.at B.before 选A还

Will you be back ____five in the afternoon?A.at B.before 选A还是B?理由是什么?
一道比较纠结的英语题,最好是清楚一点
后面的答语忘了……
还有一句是：I'm not sure ,Maybe later than that.