Your network contains a DNS server named DNS1 that runs Windows Server 2008 R2. DNS1 hosts a zone named contoso.com.Dynamic updates are enabled on the zone.The network contains a server named Web1.contoso.com.All computers on the network are configured to use DNS1 as their DNS server.You need to ensure that users can connect to Web1.contoso.com by using the name portal.contoso.com.The solution must ensure that the users can connect by using portal. contoso.com if the IP address of Web1.contoso.com changes.Which type of resource record should you create？（）