安全技术检查工作具有行政执法的性质，所以安全技术检查部门属于行政机关。（）
参考答案：错
关于声现象，下列说法正确的是：[ ]
A．声音的传播必须依靠介质
B．声音传播有时不需要介质
C．声音在水中传播的最快
D．真空可以传播声音
Only when all the fish in the lake died _____ how serious the pollution was.[ ]
A. did the villagers realise
B. the villagers realised
C. had the villagers realised
D. the villagers had realised
