Which option is more reasonable for the scope of 1+ 1 protection on the client side?

Which option is more reasonable for the scope of 1+ 1 protection on the client side?
A . Protect circuit boards, optical layer boards and line optical fibers from obstructions
B . Including the wavelength combining and demultiplexing board and the subsequent line side obstacles
C . Line-side optical fiber
D . Provide end-to-end protection starting from the client side

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Answer: D