问题：

[单选]

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）{...}