What code， inserted at line 21， is called prior to a session being migrated to a different JVM assuming the web application containing MyExample is deployed in a container which supports distributed applications？（）

• A、 public void valueUnbound（HttpSessionEvent ev）{...}
• B、 public void sessionPassivated（HttpSessionEvent ev）{...}
• C、 public void sessionDidActivate（HttpSessionEvent ev）{...}
• D、 public void sessionWillPassivate（HttpSessionEvent ev）{...}