引起外因火灾的火源有：明火、（）、（）、和（）。
参考答案：放炮火花；摩擦火机电火花；瓦斯煤尘爆炸
以下不属于三视图的是（ ）
A．正视图
B．侧视图
C．后视图
D．俯视图
—Did Linda see the traffic accident?
—No, no sooner ____ than it happened. [ ]
A. she had left
B. she left
C. did she leave
D. had she left
