只用一种试剂就能区别开硫酸铜、氯化铁、氯化镁、氯化钠四种物质，这种试剂是 [ ]
A.硝酸银溶液
B.盐酸溶液
C.氯化钡溶液
D.氢氧化钠溶液
答案：D
Not until he left home ________ the importance of his family.
A．he realised
B．did he realise
C．he did realise
D．realised he
在汶川大地震中，用到不少推土机来清理因地震堵塞的道路．推土机的履带很粗糙，又很笨重，这是为了增大______和______来增大摩擦力．
