<img src="https://image.zhihuishu.com/zhs/doctrans/docx2html/202011/052081f78b734e3d9cb762ec9a498ff0.png" />69.如图所示，一中空的无限长圆柱形导体壳层（<img src="https://image.zhihuishu.com/zhs/doctrans/docx2html/202011/d05cde406fcf427991cf931c89d700ec.png" />区域是空的），导体的内外半径分别为 a 和 b 。设导体内平行于轴线方向上通有电流 I ，且电流均匀分布在导体的横截面上。若 r 为离开导体轴线的距离，下列说法正确的是（ ） A: <img src="https://image.zhihuishu.com/zhs/doctrans/docx2html/202011/795f791319374ec1a99675dcf97c4bd6.png" />区域的磁感应强度<img src="https://image.zhihuishu.com/zhs/doctrans/docx2html/202011/76c3687faa674bfca0a28dc521ef022d.png" /> B: 导体横截面上电流密度大小为<img src="https://image.zhihuishu.com/zhs/doctrans/docx2html/202011/6b24f37b663d49fdac38a06d257c7ebf.png" /> C: <img src="https://image.zhihuishu.com/zhs/doctrans/docx2html/202011/d05cde406fcf427991cf931c89d700ec.png" />区域的磁感应强度<img src="https://image.zhihuishu.com/zhs/doctrans/docx2html/202011/d9343d6603034a18a1402d79e39534e7.png" /> D: <img src="https://image.zhihuishu.com/zhs/doctrans/docx2html/202011/54f37f9ca2dd422da5a79a1754492564.png" />区域的磁感应强度<img src="https://image.zhihuishu.com/zhs/doctrans/docx2html/202011/069097119b8441328933c47f6cb53988.png" />