Insertion loss

Which of the following multiple insertion loss descriptions is correct? ()
A. Insertion loss refers to the optical power loss at the output end of the optical signal after passing through the corresponding optical channel in the optical multiplexing device
B. Insertion loss is the performance index of a single physical channel in a multiplexer or splitter
C. In the insertion loss test, the insertion loss of each optical channel in the demultiplexer should be the same, so it is necessary to test one of the optical channels
D. In the insertion loss test of the splitter, you can use the optical signal generated by the instrument to input the input port of the splitter and test the input optical power, and then test the optical power at the output of the splitter, and the difference Insertion loss

Thank you!