How does an application-layer firewall work?

A.
examines the data in all network packets at the application layer and maintains complete connection state and sequencing information

B.
operates at Layers 3, 4 and 5, and keeps track of the actual application communication process by using an application table

C.
determines whether the connection between two applications is valid according to configurable rules

D.
allows an application on your private network that does not have a valid registered IP address to communicate with other applications through the Internet