where some of the ports are blocked by running spanning tree on a switch. When are BPDUs sent out on blocked ports?

Consider a redundant switched environment where some of the ports are blocked by running spanning tree on a switch. When are BPDUs sent out on blocked ports?

A.
Only when calculating the spanning tree

B.
Every time the switch is rebooted

C.
BPDUs are always sent out blocked ports

D.
BPDUs are never sent out blocked ports