SDH网中不采用APS协议的自愈环有（）
A.二纤单向通道倒换环
B.二纤单向复用段倒换环
C.四纤双向复用段倒换环
D.二纤双向复用段倒换环
参考答案：A
资料1
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资料3
请根据以上资料，选择以下栏目的正确选项：
“合同协议号”栏：（）。
Somebody rang my doorbell just now, but I didn't know _____. [ ]
A. who were they
B. who they were
C. who it was
