Performing a DNS lookup with dig results in this answer:

A . There is no . after linuserv.example.net in the PTR record in the forward lookup zone file
B . There is no . after linuserv in the PTR record in the forward lookup zone file
C . There is no . after linuserv.example.net in the PTR record in the reverse lookup zone file
D . The . in the NS definition in the reverse lookup zone has to be removed

Answer: C