On UNIX OS, for which of the following I/O devices the access is not via a special file of type "character I/O"?

On UNIX OS, for which of the following I/O devices the access is not via a special file of type "character I/O"? 1. Mouse 2. Screen 3. Disk On Key (USB) 4. Printer 5. None of the above This question is from a test on Operation System course. I chose answer number 5 but my teacher says that the only correct answer is answer number 3. Is there a special case where answer number 3 on USB falls? I would be happy for an edge case so they can accept my appeal please and I will not fail. Thanks in advance.