One day Tom and I were walking down the street while we saw an old man fall off his bike.

问题补充：

One day Tom and I were walking down the street while we saw an old man fall off his bike. He was badly hurt. Many people crowded around us. Someone suggested carrying the old man to the roadside, so Tom disagreed. He has learned first aid. He said that they ought to leave him where he was and checked him first. At that time the old man wasn breathing. Tom asked me to call in the First Aid Center at once and he began to start the old mans breathing, use the mouth-to-mouth way. Within five minutes, the old man began to move little and doctors also arrived. They said that we had done was correct. I have learned from this experience that a knowledge of first aid can make a really difference.while—when us—him so-but has-had checked-check call in 去掉use-using

答案：

(答案→)while—when us—him so-but has-had checked-check call in 去掉use-using

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