who 或 which 開頭的疑問句，句中關係代名詞必須用 that，才可避免和疑問詞
who/which 混淆。
例句：剛剛打電話來的那個女人是誰？
(1) Who is the woman that just called? (o)
(2) Who is the woman who just called? (x)
因先行詞為 the woman，故連接兩句時需用「人主詞性質」的關係代名詞，前有疑問詞
who，為避免混淆，不可用關係代名詞 who，(2) 為錯誤的句子，必須用關係代名詞
that [見(1) ]。
http://www.learnerhall.org/2012/10/blog-post3002.html
台灣的文法書常常這樣規定，可是閱讀了很多本英美出版的文法書都沒有看到這樣的
規定。
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?
(英國教師寫的句子，who開頭的疑問句，句中關係代名詞依然用who)