通信管道的类型有哪些?
参考答案:
1)水泥管材。2)钢管。3)高密度聚乙烯硅芯塑料管(硅芯管)。4)玻璃纤维增强塑料管。5)玻璃纤维增强塑料电缆管箱。6)双壁波纹管。
--How was your weekend, Rose?
--Awful! This was the first time I ________ alone at home, bored to death.
A.had left
B.had been left
C.have been left
D.have been leaving
液压阀是用以控制液压系统中液体的()、()和()等,以保证执行机构完成预定工作运动。