Which of the following could be the most likely cause behind this lack of response?

Doug is conducting a port scan of a target network. He knows that his client target network has a web server and that there is a mail server also which is up and running. Doug has been sweeping the network but has not been able to elicit any response from the remote target. Which of the following could be the most likely cause behind this lack of response? Select 4.

A.
UDP is filtered by a gateway

B.
The packet TTL value is too low and cannot reach the target

C.
The host might be down

D.
The destination network might be down

E.
The TCP windows size does not match

F.
ICMP is filtered by a gateway

Explanation:
If the destination host or the destination network is down there is no way to get an answer and if TTL (Time To Live) is set too low the UDP packets will "die" before reaching the host because of too many hops between the scanning computer and the target. The TCP receive window size is the amount of received data (in bytes) that can be buffered during a connection. The sending host can send only that amount of data before it must wait for an acknowledgment and window update from the receiving host and ICMP is mainly used for echo requests and not in port scans.