It’s a configuration that experts are calling a "firewall sandwich，" with the second firewall providinga second level of load balancing after traffic down. What is meant by the term "firewall sandwich"？（）

• A、 single layer of firewalling
• B、 multiple layers of firewalling
• C、 firewall connections in either an active or standby state
• D、 an architecture in which all traffic between firewalls goes through application-specific servers