铸模及铸芯用粘合剂

计算：（3x）²=______．

— Is this the first time you ______ Chengdu ?  

— No. But the first time I ______ here, the city wasn't so beautiful. [ ]

A. visited; came

B. visited; have come

C. have visited; have come

D. have visited; came

电网发生振荡时，距离保护三段可能误动

旗国法

依据《民用航空器事故和飞行事故征候调查规定》，事故发生单位应当在事发后( )h内以书面形式向事发所在地的地区管理局报告。

A．6
B．12
C．24
D．36