问题：

[单选]

You are the network administrator for your company. The network consists of a single Active Directory domain. 
The company has remote users in the sales department who work from home. The remote users’ client computers run Windows XP Professional， and they are not members of the domain. The remote users’ client computers have local Internet access through an ISP.  
The company is deploying a Windows Server 2003 computer named Server1 that has Routing and Remote Access installed. Server1 will function as a VPN server， and the remote users will use it to connect to the company network. Confidential research data will be transmitted from the remote users’ client computers. Security is critical to the company and Server1 must protect the remote users’ data transmissions to the main office. The remote client computers will use L2TP/IPSec to connect to the VPN server.  You need to choose a secure authentication method. 
 What should you do？ （）

A .  Use the authentication method of the default IPSec policies.
B .  Create a custom IPSec policy and use the Kerberos version 5 authentication protocol.
C .  Create a custom IPSec policy and use certificate-based authentication.
D .  Create a custom IPSec policy and use preshared key authentication.
E .  Use the authentication method of the Routing and Remote Access custom IPSec policy for L2TP connection.