单三形式?
"who" 是代名词,用来代替一个人或许多人都可以:"The pronoun who,in the English language,is the interrogative and relative pronoun that is used to refer to human beings."
(1) "He is someone who is a great guy."(单)
(2)"Who are they?" (复数)
(3) "Who are those students in blue uniforms?"(复数)