下列矩阵是正定矩阵的是( ) A. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/71a9153e8634bcd8779f7102fc8b1325.jpg"/> <br/> B. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/8f3e2e0c5f6fc208bf2ed7483ed65d85.jpg"/> <br/> C. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/aec8fbd217b5116261467cc9f9e42188.jpg"/> <br/> D. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/dff8adb25d8a996afcb4998d063f1847.jpg"/>