氢气在空气中的爆炸极限是()至()。
参考答案:4%;75%
胶体区别于其他分散系的本质特征是( )
A.能产生丁达尔效应
B.胶体颗粒做布朗运动
C.胶体微粒带电荷
D.分散质直径在1nm~100nm之间
—_____ are the Olympic Games held, do you know?
—Every four years. [ ]
A. How soon
B. How far
C. How long
D. How often