切断正常供电电源，检查火灾应急照明和疏散指示标志是否在内自动转换进入点亮状态。
A、5s
B、15s
C、30s
D、50s
参考答案：A
为了绿化城市，某街道要栽种一批树苗，这批树苗的成活率是80%～90%，如果要保证成活720棵，至少要栽种（ ）棵．
A．648
B．900
C．800
D．576
What happened to the city after the mass move
A．Housing costs decreased.
B．Crime rate was much lower than before.
C．All business moved out.
D．Cities declined.
