Which two tasks should you perform?

Your network has Lync Server 2010 and Microsoft Office Communications Server 2007 R2 deployed.
The network contains an Edge Server that runs Office Communications Server 2007 R2.
Lync users report that they are unable to initiate IM conversations to federated contacts.
You verify that Office Communications Server 2007 R2 users can successfully initiate IM conversations to federated contacts.
You need to ensure that all Lync users can successfully communicate with the federated contacts.
Which two tasks should you perform? (Each correct answer presents part of the solution. Choose two.)

A.
From the Lync Server 2010 Control Panel, modify the Provider settings.

B.
From the Lync Server 2010 Control Panel, create an External Access Policy.

C.
From the Lync Server 2010 Topology Builder, add a Mediation Server, and then publish the topology.

D.
From the Lync Server 2010 Topology Builder, configure the Site Federation Route Assignment, and then publish the topology.

Explanation:
http://technet.microsoft.com/en-us/library/gg182586.aspx
http://technet.microsoft.com/en-us/library/gg425785.aspx

http://technet.microsoft.com/en-us/library/gg425908.aspx

http://www.ocspedia.com/edge_server/Microsoft_Lync_Server_2010_Access_Edge_Policy.aspx?ArticleID=109

To use federation and public IM connectivity, you must enable federation and public IM connectivity at the global level.