Close activity started with onActivityResult when a C2DM message is received

In my application here is the scenario

1. Activity A receives a regiID from C2DM
2. Activity A sends regID to server S
3. Activity A calls activity B through onActivityResult()
4. While the user is in activity B, the server S sends a "close" message through C2DM
5. Activity A re开发者_开发技巧ceives the message

I want to close activity A and B at this time (however activity B is in foreground). How can I achieve this? [by close i mean finish]

Please guide