建立完整的维护资料是（）的基本内容之一。
参考答案：本地传送网SDH设备维护
具有协同清除血循环中免疫复合物功能的细胞（）.
A.中性粒细胞
B.红细胞
C.NK细胞
D.巨噬细胞
E.嗜碱性粒细胞
What was he doing ______ his father _______ back?
A. while; came B. when; came C. when; was coming
