正常情况下，锅炉停炉（）小时后，方可开启人孔门及风机档板，进行自然通风冷却。
A.4-6
B.2-4
C.1-2
D.24
参考答案：A
Great changes_____ in the small city in the last 20 years.
A．have taken place
B．took place
C．had taken place
D．are happening
梅兰日兰U86/L86开关与插座的正常操作寿命各是多久？
