On a machine running several X servers, how are the different instances of the X11 server identified?

On a machine running several X servers, how are the different instances of the X11 server identified?
A . By a fixed UUID that is defined in the X11 configuration file.
B . By a unique IPv6 address from the fe80:: /64 subnet.
C . By the name of the user that runs the X server like x11: bob.
D . By a device name like /dev/X11/xservers/1.
E . By a display name like: 1.

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Answer: E