http://image.zhihuishu.com/zhs/onlineexam/ueditor/201801/8b154e380bdd4676a37360d210a69923.png

http://image.zhihuishu.com/zhs/onlineexam/ueditor/201801/532cfa196ddd479aaaa30c9c02c23e57.png

http://image.zhihuishu.com/zhs/onlineexam/ueditor/201801/634d52b7ac1341c0bbd190eacb473e23.png

http://image.zhihuishu.com/zhs/onlineexam/ueditor/201801/ae30d7c9a9be438aa2ecc5e05e2383f3.png

直径为d的圆截面钢杆受轴向拉力作用，已知其纵向线应变为<img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201801/767e7380667c4d688dc6c10aaf9ef9ca.png" />，弹性模量为E，给出杆轴力的四种答案，问哪一种是正确的？ A: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201801/8f37f5bc2dc94791b84f000092e493b3.png" /> B: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201801/073a4958352c4951b6171383bc4ca238.png" /> C: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201801/85f164582f934227b4bbb5ee0a7267dc.png" /> D: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201801/95e66fa0756f4ffd8c0ae368c0c88564.png" />