A.at a time 依次,逐一,每次.如;1.Don't try to do everything at once,take it a bit at a time.不要什么事情都一块干,要一次做一点.2.Take the pills two at a time.每次服2粒.
B.at one time一度,从前.如:At one time I used to go skiing every winter.我有一度每到冬天就去滑雪.