What protection method is used in 4F BLSR networks?（）
- A Each fiber has half of its SONET capacity reserved for protection.
- B Two fibers in the span carry working traffic, while the other two fibers in the span are reserved for protection.
- C The signal is sent simultaneously in both directions around the ring. The receiver determines which fiber to listen to based on signal quality.
- D The signal is sent simultaneously in both directions around the ring. The transmitter determines which fiber has the working bit set for the receiver to acknowledge.