8．直径为D的圆形截面，则其对形心轴的惯性矩为（   ）。

<img src、"http://anhui.ouchn.cn/pluginfile.php/56687/question/answer/642786/9/224159/01.png" alt="" width="50" height="54">

<img src、"http://anhui.ouchn.cn/pluginfile.php/56687/question/answer/642786/9/224160/02.png" alt="" width="41" height="54">

<img src、"http://anhui.ouchn.cn/pluginfile.php/56687/question/answer/642786/9/224161/03.png" alt="" width="44" height="50">

<img src、"http://anhui.ouchn.cn/pluginfile.php/56687/question/answer/642786/9/224162/04.png" alt="" width="46" height="53">