How do UDP scans work?

How do UDP scans work?
A . By establishing a three-way handshake.
B . By sending SYN packets to see what ports are open.
C . UDP scans have to rely on ICMP “port unreachable” messages to determine whether a port is open. When the scanner sends a UDP packet and the port is not open on the victim’s system, that system will respond with an ICMP “port unreachable” message.
D . By sending ICMP “port unreachable” messages to the victim.

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Answer: C