重点讲解：
1. all of a sudden 突然地；出乎意料地；
例句：All of a sudden she didn't look sleepy any more.
她突然看起来一点都不困了WvwTh|IbII]oY。
2. pay off 偿清，还清（债务）；
例句：It would take him the rest of his life to pay off that loan.
他得用余生来还清那笔贷款T4.HIV#^r8BhJ[Nq40mk。
3. lead to 招致；致使；导致；
例句：A lack of prudence may lead to financial problems.
不够谨慎可能会导致财政上出现问题u+fUDa_Q0otEg8_2N=H。
4. be responsible for 负有责任的；应承担责任的；
例句：He could never quite put his finger on who or what was responsible for all this.
他一直就搞不清楚是谁或者什么造成了这一切=oCM|bSQpezknwZ|h。