Which of the following facts determines how often a nonroot bridge or switch sends an 802.1D STP Hello BPDU messages?
a. The Hello timer as configured on that switch
b. The Hello timer as configured on the root switch
c. It is always every 2 seconds
d. The switch reacts to BPDUs received from the root switch by sending another BPDU 2 seconds after receiving the BPDU