When Cisco ACI connects to an outside Layers 2 network, where does the ACI fabric flood the STP BPDU frame?

When Cisco ACI connects to an outside Layers 2 network, where does the ACI fabric flood the STP BPDU frame?
A. within the bridge domain
B. within the APIC
C. within the access encap VLAN
D. between all the spine and leaf switches