How does a switch with MSTP enabled learn the boundary between its MST region and another MST region?

A.
It receives a BPDU from a non-Root switch with a Bridge Priority identical to the Root Bridge Priority.

B.
It receives a BPDU from a switch with a differentMST configuration.

C.
It sends a BPDU that is rejected by a neighboringswitch.

D.
It receives a BPDU from a switch with a MAC address that is not in its MSTP forwarding table.