An organization finds unauthorized software has been installed on a number of workstations. The software was found to contain a Trojan which had been uploading data to an unknown external party. Which of the following would have BEST prevented the installation of the unauthorized software?

A. Implementing application blacklisting
B. Implementing an intrusion detection system (IDS)
C. Banning executable file downloads at the Internet firewall
D. Removing local administrator rights