问题：

[单选] What protection method is used in 4F MS-SPRing networks？（）

A .  Each fiber has half of its capacity reserved for protection.
B .  Two fibers carry their full capacities while the other two fibers 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.