如图所示，圆形截面区域内存在着与截面相垂直的磁场，磁感应强度随时间变化．磁场区域外有一与圆形截面共面的矩形导体回路 <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201804/0eea13cd51b54af3b701a0035e6416fe.png" />，以<img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201804/865048ee310d46e09b743d6cfc93fa4a.png" />表示在导体<img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201804/55379797515a41b88e0dcb947c44b585.png" />段上产生的感生电动势， I 表示回路中的感应电流，则（ ）<img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201804/95d31735160f47dcb4d8f5ac973c0ce8.png" /> A: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201804/986a62c34302465cad899f00080acc4f.png" /> B: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201804/4b6ab53aeebc4f92b722d8fff6237a87.png" /> C: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201804/d869de90826645c1b386e2019aa86be2.png" /> D: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201804/da8edb3a9af0442bb11dcee16f061375.png" />