What is meant by the term "firewall sandwich"？（） A: multiple layers of firewalling B: a method of operating firewalls from multiple vendors C: firewall connections in either an active or standby state D: an architecture in which all traffic between firewalls goes through application-specific servers E: an architecture in which all traffic between firewalls goes through application-specific gateway

What is meant by the term "firewall sandwich"？（） A: multiple layers of firewalling B: a method of operating firewalls from multiple vendors C: firewall connections in either an active or standby state D: an architecture in which all traffic between firewalls goes through application-specific servers E: an architecture in which all traffic between firewalls goes through application-specific gateways