@tanguatlay,
tanguatlay wrote:Could you please explain why the second sentence in each case is wrong?
Let me! Questions of the form "who [verb phrase]" always use the third person singular of the verb, even if it is possible that the answer may be about more than one person.
Teacher: Who knows the capital of Japan?
Multiple children: I do!
Q: Who is the boss in this office?
A: Joe Smith.
Q: Who is working late today?
A: John, Mary and Anne.
However who are... questions are possible e.g. who are the people in this photograph? Who are the men in that car? Who are the children playing ball?