What can you infer from this observation?

While scanning a network you observe that all of the web servers in the DMZ are responding to ACK packets on port 80. What can you infer from this observation?

A.
They are using UNIX based web servers

B.
They are using Windows based web servers

C.
They are not using an Intrusion Detection System

D.
They are not using a stateful inspection firewall

Explanation:
If they used a stateful inspection firewall this firewall would know if there has been a SYN-ACK before the ACK.