SDH、PTN接入环的拓扑结构指标包含哪些（）。
A.超大环
B.接入层双归
C.成环率
D.长支链
参考答案：A, B, C, D
—What are you doing now?
—I am writing to my cousin. He moved to America two weeks ago.
He must be ______ receiving my e-mail. [ ]
A. listening to
B. looking forward to
C. hoping to
在工单处理时，工单类型和状态需选择（）
A.母单、错单
B.子单、错单
C.母单、待人工和错单
D.子单、待人工和错单
