back up和back off

俩个都有后退的意思?那有什么不同

back off :滚开,后退。比较口语化,用于辱骂等命令语气。
back up:交通堵塞。如traffic backs up around the corner...写作时可用这句,不要用traffic is busy...
back up还有倒退之意The economy has been always backing up(off)....经济持续倒退。这里也可用back off.
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