从社会功能的角度来看，“警察”系指（ ）
A．警察机关
B．警察人员
C．警察作用
D．警察行为
参考答案：C
哪些船舶必须设置独立的应急电源？
It ____ last night, for the ground is wet now.
A．might not have rained
B．must have rained
C．couldn't have rained
D．rained
