B、不存在
C、<img src="https://s3.cn-north-1.amazonaws.com.cn/qingshuxuetang/examination/imgUplod/1528952759176b346699d8d784127b96f31c2a78ba792image18.png">
D、<img src="https://s3.cn-north-1.amazonaws.com.cn/qingshuxuetang/examination/imgUplod/15289527592465f073190ee334195a13cc60150354e48image19.png">