一根质量可以忽略不计的轻弹簧，某人用两手握住它的两端，分别用100N的力向两边拉，弹簧伸长4cm，则这弹簧的劲度系数k=____N/m．

玉米是重要的粮食作物．下图表示玉米植物体的结构层次，请据图回答：

关于声现象，下列说法正确的是（    ）

根据句意用所给单词的适当形式填空。

Everyone should behave ____  in public. (polite)

 --Who's that boy standing over there?  --Sam, a foreign friend of____ . (I) 

After working all night, Kate couldn't stay____ the next morning. (wake)

The UK is an old ____ country, where there are many palaces and castles. (Europe)

Nothing would prevent UNICEF, an international charity,____ the living standards of the children in poor areas. (raise)