All the web servers in the DMZ respond to ACK scan on port 80. Why is this happening ?

A.
They are all Windows based webserver

B.
They are all Unix based webserver

C.
The company is not using IDS

D.
The company is not using a stateful firewall

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