题目5单选题
矢量场<img src="https://tihai-oss-cloud.itihey.com/img/0b91674493b34bac663ccdb052765be5.png">在闭合路径<img src="https://tihai-oss-cloud.itihey.com/img/d913f4c30b8db2f7c9088512b7d3f9ba.png">的环流定义为( )__A. <img src="https://tihai-oss-cloud.itihey.com/img/568e2fa2fe35b8030cafe67852bc251c.png">B. <img src="https://tihai-oss-cloud.itihey.com/img/83f33e9355fd1b5463c074f85eb9ad01.png">C. <img src="https://tihai-oss-cloud.itihey.com/img/308bc6fe43ec9a238f4b3ca309bc3ecc.png">D. <img src="https://tihai-oss-cloud.itihey.com/img/46d8e4b16622966547420297cc827c3b.png">