Your network consists of a single Active Directory domain. All domain controllers run Windows Server 2008 R2. The network contains 100 servers and 5,000 client computers. The client computers run either Windows XP Service Pack 1 or Windows 7. You need to plan a VPN solution that meets the following requirements:   èStores VPN passwords as encrypted text  èSupports Suite B cryptographic algorithms èSupports automatic enrollment of certificates   èSupports client computers that are configured as members of a workgroup What should you include in your plan?（） 

• A、Upgrade the client computers to Windows XP Service Pack 3. Implement a stand-alone certification authority （CA）. Implement an IPsec VPN that uses certificate-based authentication.
• B、Upgrade the client computers to Windows XP Service Pack 3. Implement an enterprise certification authority （CA） that is based on Windows Server？2008 R2. Implement an IPsec VPN that uses Kerberos  authentication.
• C、Upgrade the client computers to Windows 7. Implement an enterprise certification authority （CA） that is  based on Windows Server 2008 R2. Implement an IPsec VPN that uses pre-shared keys.
• D、Upgrade the client computers to Windows 7. Implement an enterprise certification authority （CA） that is  based on Windows Server 2008 R2. Implement an IPsec VPN that uses certificate-based authentication.