pay off释义：
付清取得成功贿赂报复
例句：
I need to pay off all my debts before I leave the country.
我得在离开该国前偿清所有债务。
词组：
payoff function支付函数
视频：
场景会话
pay back释义：
vt. 偿还报答
例句：
He burst into tears， begging her to forgive him and swearing to pay back everything he had stolen.
他突然哭了起来，求她原谅他并发誓要偿还他偷的所有东西。
词组：
payback period回收期偿还期
payback period of investment投资回收期
视频：
场景会话
