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重点讲解：
1. now and then 时而；有时；
Now and then they heard the roar of a heavy truck.
他们不时地听到大卡车的轰鸣声。
2. go down （价格、水平或数量）下降，下跌；
Crime has gone down 70 percent.
犯罪率下降了70%。
3. a number of 若干；许多；
This equipment has saved the lives of a number of new born children.
这套设备已经挽救了许多新生儿的生命。
4. learn from （从…中）吸取（教训）；
I am convinced that he has learned from his mistakes.
我深信他已从错误中吸取了教训。
