You are the network administrator for your company. The network consists of a single Active Directory domain with three sites named Site1， Site2， and Site3. The sites and site links are configured to use Site2 to connect Site1 and Site3. Each site contains three Windows Server 2003 domain controllers. A domaincontroller in each site is configured as a preferred bridgehead server. All user and group accounts are created in Site1. Several new users start work in Site2. When they attempt to log on to the network， the logon fails. You confirm that the user accounts are created and are visible in Site1 and Site2. You discover that the preferred IP bridgehead server in Site2 failed. You repair the server and confirm that replication is successful to Site2. You need to ensure that the failure of a single domain controller in any site will not interfere with Active Directory replication between sites.  What are two possible ways to achieve this goal？（）

• A、 Configure an IP site link between Site1 and Site3.
• B、 Configure two domain controllers in each site as preferred IP bridgehead servers.
• C、 Configure two domain controllers in each site as preferred SMTP bridgehead servers.
• D、 Configure each site to have no preferred bridgehead servers.
• E、 Configure an SMTP site link between each of the sites. Assign a cost of 200 to the SMTP site link.