设平面区域<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/528f0604e4ec4256a7ae38ae783a34ac.png?x-oss-process=image/sharpen,200"/>为单连通的开区域，函数<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/2d676ecd7fba4605ba4fa451e0f0312e.png?x-oss-process=image/sharpen,200"/>在<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/d3a661abc0044a91abe0d2871523d253.png?x-oss-process=image/sharpen,200"/>内具有一阶连续偏导数，则曲线积分<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/b9123b597d5241fb98bfa381dd264902.png?x-oss-process=image/sharpen,200"/>在<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/528f0604e4ec4256a7ae38ae783a34ac.png?x-oss-process=image/sharpen,200"/>内与路径无关的充分必要条件是（）<br/>