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