下列矩阵中不是正定矩阵的为( ).<br/> A、<img src="https://static.zxtiku.com/wangke/imgs/topic/8dc2f7eeb838ec9734a1f00c07db228f.jpg"/><br/> B、<img src="https://static.zxtiku.com/wangke/imgs/topic/9edf6461e961903dfd73336171883e04.jpg"/><br/> C、<img src="https://static.zxtiku.com/wangke/imgs/topic/7df9b84a43d78bca697b92d91d35c2cf.jpg"/><br/> D、<img src="https://static.zxtiku.com/wangke/imgs/topic/a8cfa62b3660019df3a276aa27aa51c2.jpg"/>