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当前学科：中级无损检测技术资格人员
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220KV的X射线，对钢和铜进行射线照相，两者的厚度当量系数为1比1.4，若以28mm厚的钢板进行射线照相时的曝光条件，对铜进行射线照相，则此铜板的厚度应该是多少？（）
A . A、20mm
B . B、34mm
C . C、16.8mm
D . D、25mm
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