No one can walk the wire in the air without a bit of fear unless very young. A. having tr

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A. having trained
(答案→)B. trained
C. to be trained
D. being trained
解析：
这题考查条件状语从句的省略，英语中，如果主从句的主语一致，可以省略从句中的主语和be动词，像这题，完整的表达应该是：unless he is trained young，省略he is后就剩下trained，选B。