影响几何不清晰度的主要原因是（）、（）、（）至胶片的距离和焦点至缺陷的距离。
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如果某类风险发生损失的频率很高，但损失程度不是很大，一般建议采取（）的方法。
A. 风险回避
B. 风险转移
C. 风险自留
D. 保险
Which of the following is not a recommended safe practice?（）
A. Securing equipment against slipping or drifting
B. Operating machinery at its recommended speed
C. Repairing loose handles on tools before using
D. Using tools for purpose for which they are not designed
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