题目5单选题
真空中一根无限长直细导线上通电流I,则距导线垂直距离为a的空间某点处的磁能密度为A. <img src="https://tihai-oss-cloud.itihey.com/img/b28abe830be8d7a106a1bbd0b857df06.png">B. <img src="https://tihai-oss-cloud.itihey.com/img/0274623b4b13889a9b36ea7b1be3c319.png">C. <img src="https://tihai-oss-cloud.itihey.com/img/24b7d7f54baa93a368dce3c02130d918.png">D. <img src="https://tihai-oss-cloud.itihey.com/img/125093c942f1441e46c6221ed99b37a7.png">