感染是慢性支气管炎发生发展的什么因素（）
A.唯一因素
B.重要因素
C.首发因素
D.次要因素
E.无关因素
参考答案：B
Where does the cloud come from?[ ]
A. It comes from the clouds.
B. It comes from the vapour.
C. It comes from the water.
D. It comes from the water in the river.
什么是发电机轴电压？
