Which native mechanism does OSPF use to prevent loops in MPLS VPNs?

Which native mechanism does OSPF use to prevent loops in MPLS VPNs?
A . CE devices that run OSPF set the DN bit toward the PE router
B . PE devices that run OSPF clear the DN bit toward the CE router
C . CE devices that run OSPF clear the DN bit toward the PE router
D . Creation of PE to PE OSPF sham link across the MPLS-created super backbone
E . PE routers verify OSPF domain IDs used by CE OSPF processes
F . PE devices that run OSPF set the DN bit toward the CE router

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Answer: F