I 've told him several times but he ---- doesn't understand

I 've told him several times but he ____ doesn't understand
A yet
B still
不是说still 用在肯定句中.yet 用在否定句中吗?
那个这道题有doesn't ,应该是否定句啊,不是应该选yet
为什么使用still
这里的否定句并不是指doesn't underestand
中的doesn't ,可以把doesn't underestand 看成一个词组,这样的话这句话就变成了肯定句,如果not 在yet 前面的话（句子中,而不是某个词的联合体）,那么就是否定句.是这样的吧?