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