流行病学研究疾病的范围是（）
A.慢性病
B.急性病
C.流行病
D.传染病
E.疾病和健康状况
参考答案：E
瓦斯发生爆炸的条件是（）。
A.瓦斯浓度达到5%—16%
B.空气中氧气浓度在12%以上
C.有足够能量的点火源
D.有煤尘存在
You'd better leave , because it's dangerous here. [ ]
A. as possible as you can
B. as quick as possible
C. as later as you can
D. as soon as possible
