If a switch port goes into a blocked state only when a superior BPDU is received, what mechanism must be in use?

If a switch port goes into a blocked state only when a superior BPDU is received, what mechanism must be in use?
A . STP root guard
B . EtherChannel guard
C . loop guard
D . STP BPDU guard

Answer: A

Explanation:

Root guard allows the device to participate in STP as long as the device does not try to become the root. If root guard blocks the port, subsequent recovery is automatic. Recovery occurs as soon as the offending device ceases to send superior BPDUs.

Source: http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10588-74.html