An administrator needs to add the /usr/vac/bin directory to their PATH environment variable for the current session.  Which of the following will accomplish this task（）

• A、path=$path：/usr/vac/bin； export path
• B、path.=/usr/vac/bin； export $path
• C、Modify the path environment variable in /etc/profile
• D、Modify the path environment variable in /.profile