函数<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/7b9cb496aac74aa4a5a24c5b6117f38d.png?x-oss-process=image/sharpen,200"/>在点<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/ee93638def124c038c4c46ebc7f40370.png?x-oss-process=image/sharpen,200"/>处具有所有二阶连续偏导数，且<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/78d20d867bf84509b7c78b262f264b40.png?x-oss-process=image/sharpen,200"/>，则下列哪个条件可以保证函数在点<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/ee93638def124c038c4c46ebc7f40370.png?x-oss-process=image/sharpen,200"/>处不取极值