Which of the following could cause this?

An administrator is trying to make a hard link to an ordinary file but ln returns an error. Which of the
following could cause this?

A.
The source file is hidden.

B.
The source file is read-only.

C.
The source file is a shell script.

D.
The administrator does not own the source file.

E.
The source and the target are on different filesystems.