How do switches participating in an STP network become aware of topology changes?

A. The root bridge is responsible for sensing the change and sending Topology Change Notification BPDUs.
B. Each switch is responsible for sensing the change and sending Topology Change Notification BPDUs.
C. The root bridge polls each switch participating in STP for changes.
D. The switches participating in STP poll the root bridge for changes.

B. Each switch is responsible for sensing changes to the topology. Whenever the topology changes, a Topology Change Notification (TCN) is sent out all root ports and an acknowledgment is sent back. This happens until the root bridge sends back a notification.