Which file does the who command read to obtain its output?

A user contacts the help desk to report that a script run by the user’s department is reporting an
error. The script checks for who is logged in. Your first action is to make sure that the file that the
who command reads for its output, is present.
Which file does the who command read to obtain its output?

A.
/var/adm/users

B.
/var/adm/utmpx