The Cisco network-based virtual firewall service solution helps service providers to deliver cost-effective， scalable， integrated security services for enterprise customers using Cisco platforms.What is a virtual firewall？（）

• A、 another name for a firewall deployed in routed mode
• B、 another name for a firewall deployed in transparent mode
• C、 a separation of multiple firewall security contexts on a single firewall
• D、 a firewall that， when deployed in routed mode， can support up to 1000 VLANs per context