(國中英文一問) Who + (單or複數) 動詞

Exercise: Who ( ) this?

They do it.

(A). Does

(B). Do

答案是 (A). 原因為何?

是因為 “當主詞被疑問詞代替 一律視為單數" 嗎? (Who are you is an exception case.) Please be advised. Thanks.