A company has recently switched ISPs and is being assigned a new block of public addresses.  The public web and FTP servers must be re-addressed to support this change.  After changing IP addresses and updating the DNS records many customers have reported that they are not able to access the web or FTP servers.  Which of the following could have been done prior to making these changes to make it more transparent to users？（）

A.Reduce the TTL on the DHCP records.
B.Configure a primary DNS server for the PTR records.
C.Reduce the TTL on the DNS records.
D.Configure a caching only DNS server.