As a securing consultant, what are some of the things you would recommend to a company to ensure DNS security?

As a securing consultant, what are some of the things you would recommend to a company to ensure DNS security?
A . Use the same machines for DNS and other applications
B. Harden DNS servers
C. Use split-horizon operation for DNS servers
D. Restrict Zone transfers
E. Have subnet diversity between DNS servers

Answer: B,C,D,E