Why is a transparent firewall considered a "bump in the wire" or "stealth firewall"?

Why is a transparent firewall considered a "bump in the wire" or "stealth firewall"?
A . It is a bump on the wire because it filters packets at the OSI physical layer.
B . Using separate IP subnets for inside and outside interfaces, packets are inspected quietly.
C . A transparent firewall is considered a Layer 2 firewall and has no routing capabilities.
D . Transparent firewalls work in "stealth mode" by using network taps.
E . Because transparent firewalls can route between inside and outside interfaces, packets are "bumped" between them by using special firewall rule-sets.

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Answer: C