While /tmp/file contains data, why is /tmp/file empty afterwards?

When running the command

sed -e "s/a/b/" /tmp/file >/tmp/file

While /tmp/file contains data, why is /tmp/file empty afterwards?
A . The file order is incorrect. The destination file must be mentioned before the command to ensure redirection.
B . The command sed did not match anything in that file therefore the output is empty.
C . When the shell establishes the redirection it overwrites the target file before the redirected command starts and opens it for reading.
D . Redirection for shell commands do not work using the > character. It only works using the | character instead.

View Answer

Answer: C