From a local machine, connectivity with a server named server1.comptia.org is tested. It cannot be reached, but it can be accessed via the IP address. The DNS server is checked and everything seems to be functioning correctly. Which of the following is the MOST effective command to use to resolve the problem?

A.
ipconfig /registerdns

B.
nslookup server1.comptia.org

C.
ipconfig /renew

D.
ifconfig /flushdns

E.
ipconfig /flushdns