What is the recommended way to ensure that firewalls have the most current set of signatures for up-to-date protection?

What is the recommended way to ensure that firewalls have the most current set of signatures for up-to-date protection?
A . Run a Perl script to regularly check for updates and alert when one is released
B . Monitor update announcements and manually push updates to Crewall
C . Store updates on an intermediary server and point all the firewalls to it
D . Use dynamic updates with the most aggressive schedule required by business needs

Answer: D