In which way does the Bridge Assurance mechanism modify the default spanning-tree behavior in effort to prevent bridging loops?

In which way does the Bridge Assurance mechanism modify the default spanning-tree behavior in effort to prevent bridging loops?
A . BPDUs are sent bidirectional on all active network ports, including blocked and alternate ports
B . Extended topology information is encoded into all BPDUs
C . If BPDUs are no longer received on a port, the switch immediately sends out a TCN BPDU
D . Received BPDUs are looped back toward the sender to ensure that the link is bidirectional

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Answer: A