【单选题】
Switch R1 has been configured with the root guard feature. <br /> What statement is true if the spanning  tree enhancement Root Guard is enabled？ （）
A、 If BPDUs are not received on a non-designated port， the port is moved into the STP loop- inconsistent blocked state
B、 IF BPDUs are received on a PortFast enabled port， the port is disabled.
C、 If superior BPDUs are received on a designated port， the interface is placed into the root- inconsistent blocked state.
D、 If inferior BPDUs are received on a root port， all blocked ports become alternate paths to the root bride.
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