What are reverse DNS entries used for?

A.
Reverse DNS enable diagnostic commands like traceroute to work.

B.
Reverse DNS gives you information about the owner of the DNS entry.

C.
Reverse DNS provides the hostname for a particular numeric IP address.

D.
Reverse DNS provides geographical information about the DNS net location.

Explanation/Reference:
Since normal DNS resolves hostnames into IP adresses, RDNS resolves IP addresses into hostnames.