who 或 which 开头的疑问句，句中关系代名词必须用 that，才可避免和疑问词
who/which 混淆。
例句：刚刚打电话来的那个女人是谁？
(1) Who is the woman that just called? (o)
(2) Who is the woman who just called? (x)
因先行词为 the woman，故连接两句时需用“人主词性质”的关系代名词，前有疑问词
who，为避免混淆，不可用关系代名词 who，(2) 为错误的句子，必须用关系代名词
that [见(1) ]。
http://www.learnerhall.org/2012/10/blog-post3002.html
台湾的文法书常常这样规定，可是阅读了很多本英美出版的文法书都没有看到这样的
规定。
https://forum.wordreference.com/threads/who-is-the-man-who-that.3888112/
Who is this 'Taiwanese teacher of English' who seems to have the monopoly on
imposing their own dodgy exercises on your curriculum?
(英国教师写的句子，who开头的疑问句，句中关系代名词依然用who)