设载荷系数fw，可靠性系数fc，精度系数fa，则FC的计算公式为（    ）。

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<img src、"http://hebei.ouchn.cn/pluginfile.php/72057/question/answer/1279253/25/288294/2.png" alt="" width="75" height="45">

<img src、"http://hebei.ouchn.cn/pluginfile.php/72057/question/answer/1279253/25/288295/3.png" alt="" width="77" height="45">

<img src、"http://hebei.ouchn.cn/pluginfile.php/72057/question/answer/1279253/25/288296/4.png" alt="" width="73" height="45">