问题：

[单选] 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
E . a firewall that has multiple contexts， all of which share the same policies （such as NAT and ACLs）