英语学习资料：成本效益分析(Cost-Benefit Analysis)

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　　when someone gives us something that has a huge potential benefit—and costs us absolutely nothing—what should we say to such a fine person? “thank you!”i landed in new york feeling lonely, guilty, and ashamed of myself. i immediately called lyda and told her my cost-benefit story. i assured her: “the next time you help me with my driving, i am just going to say, ‘thank you.’”“sure you will,” she said with a laugh (sarca ... free of charge). for some reason, she seemed to doubt that i had undergone a true religious conversion.“just you wait. i am going to do better.” i continued.“we’ll see.” she replied.