Which reason would you see activity on the TURN server when a call is established between two external endpoints?

A company has enabled ICE to optimize call flows and improve video quality between their Cisco Collaboration endpoints internally and externally.
For which reason would you see activity on the TURN server when a call is established between two external endpoints?
A. The video call is using encryption, which is not supported by ICE with CUCM 12.5
B. ICE cannot reduce the packet loss on the link
C. A STUN cannot punch holes in the firewall
D. The video call is using 4K resolution, which is not supported by ICE with CUCM 12.5